I am playing D&D 4e and I have a question about how I can create an excellent pirate adventure. Since people have pirate adventures in other RPG systems, and Stack Exchange is about useful answers, should I tag my question with dungeons-and-dragons or maybe system-agnostic in order to cover a wider variety of responses and be more helpful to other people making pirate adventures, even if I'm only interested in the dnd-4e answers myself? Also, I've seen questions get closed as "too localized." Would doing this help keep my question from getting closed?
Always tag your question with the exact system you're using if you're asking a question to help you out in a problem about your system and you're only really interested in answers relevant to your system. Even if your question could be relevant to other RPG systems, or a broader category of RPG systems, ask about your exact problem in your exact RPG. There's no need to broaden your question to, say, dungeons-and-dragons and system-agnostic if you're not genuinely interested in answers broader than your own system.
If you're playing in one RPG system and want answers about creating a pirate adventure for that system, and you see a similar question asked for a different system which has answers which are not helpful to you or not what you're after, feel free to ask your question tagged with your own system.
System-specific can be much more useful to you and others than a broader scope
Even if it is an issue that could be relevant to other systems, if you're not interested in broader answers, you should just focus on getting your own problem solved, not everyone else's. Ask about the problem you're actually facing in your own system and get a solution to your specific problem in your own system. There's no need to broaden the system scope of your question unless you specifically want broader answers.
If the issue is relevant to other systems, players of those other systems can still often learn just fine from questions targeted at your own system. In my personal experience when I only played D&D, I learned some things from questions about Fate, Burning Wheel and other systems, and often the specific philosophies that those RPGs bring to the table broadened my experiences and taught me things I could bring back to my own D&D gameplay.
If players of other systems don't get the answers they need, they can ask another question scoped to their own system, or ask a broader-scoped question if they're interested in broader-scoped answers.
This is OK if it's what you're after:
Answers contain sorta-helpful stuff for your system of choice.
But this is more useful for solving system-specific issues:
Answers are targeted straight at 4e, mentioning classes and themes and monsters and terrain and other content available.
Answers are targeted straight at Ars Magica 5 and contain spell recommendations and other useful information.
Answers targeted straight at Fate core, recommending piratey aspects, what skills and stunts various pirates should have, mooks and groups and characters, and possibly some recommendations from the magic system toolkit
I would prefer to see the same kind of question asked several times and for different systems each time by users wanting answers in those specific systems, than one that attempts to deal with all possible systems and whose answers are only of limited help to anyone wanting answers for their specific systems.
You should tag a question with a specific system if you want it answered for a specific system, and you should tag more widely if you're trying to solve a wider problem.
Usually, tagging for the system you are playing is the right thing to do. You should be asking about the problem you have, and not making it more generic out of an attempt to get wider reach. If you are playing Pathfinder, there may be better specific answers to your question than you'd get by tagging with dnd-3.5e also, or d20, or dungeons-and-dragons in general.
Some people don't just play one game system, however, and there are many gaming techniques widely applicable to the majority of existing games. There's certainly a place on the site for those. You will note there is a wide variety of "system agnostic" products out there that are quite popular - generic NPCs, generic plot hooks, etc. People find these useful and are entitled to look for more generic answers.
For those that really want things hooked into a specific ruleset, that's a valid mode of play and those people can tag with a specific system tag. In the end, it is up to the question asker. Trying to dictate which they 'should' be doing is not an appropriate use of time. Retagging their question might be helpful, but if they revert you should abide by their wishes - if you want to ask that question specific to a system, you ask it.
Just like on Stack Overflow, you can ask for a generic algorithm for a sort, or for that sort in C or Python or Java. They both solve problems - different problems, for different people.
I'm not very active here. However, on other Stack Exchange sites, tags are normally used to help notify eager or knowledgeable perusers of a new question in their area of interest or expertise.
Tags are usually never used to categorize the questions nor to limit the answers desired — Puzzling being one exception there, of course.
If the person asking, or anyone else, thinks that adding a tag would be useful to help attract answers, then they do it.
Use of system-agnostic doesn't seem to be synonymous with something like any-system.
It seems to me that it is for answers which do not, at all, make reference to anything pertaining to any certain system. Usually, such an criterion would be one which the asker had added or requested.
In short: I would recommend that you do not use system-agnostic — unless you want an answer which doesn't constrict itself to any system; if you want answers of broad pertinence, whether in a specific answer or in the general slew of answers, then don't tag for any system.