We do not enforce style, only clarity, as a matter of basic SE principles
I'll quote myself from our proposed FAQ, Is there a style guide for posts?, with new emphasis:
… [spell names, &c] shouldn't be changed through editing, because both are correct and switching the style doesn't make a post even a little bit easier to read. This is similar to AmE and BrE spellings: it's not appropriate to edit someone's spelling of “realise” to “realize” or vice versa, since it wasn't wrong in the first place. As a rule of thumb, we try to be consistent with the original author's spells and spelling conventions when doing an edit.
We don't edit for style taste here for two big, important reasons:
It doesn't make posts even a little bit easier to read. This is a hardcoded reason to reject edit suggestions, which we should all remember from our time before getting full edit privileges, and be occasionally reminded of when we review the suggested edit queue. The suggested edit process teaches us how to be good editors, and how it taught us to edit is supposed to be remembered even when we're not forced anymore by the site to follow it.
It damages the experience of newer users for zero gain to the site. We have a number of metas from new users — the small percentage confident enough to speak up — posting here to ask why their style choices were rejected by higher-rep editors. They naturally assume that we know something they don't and ask what they did wrong.
These metas are responded to with reassurances that they did nothing wrong — that it was actually the higher-rep editor who did something wrong.
Don't annoy, anger, confuse, or frighten our new users for no good reason. There's enough arcane knowledge about how to use the site that they're supposed to absorb organically by using it and observing how others use it, please don't hinder their learning by making changes to their posts that are only noise, not teaching.
Therefore: Edit spell styling only when it objectively improves the post
Changing style to match an arbitrary standard which officially doesn't exist here is harm, not improvement.
If spells in a post are all lower case with no styling, and that makes it hard to read the post, then go ahead and style them using whichever arbitrary style convention is most suitable in context.
If someone wants to write “magic missile” in a D&D 3e post, that's fine. If they write “Magic Missile” in an AD&D post, that's fine too. If they want to write “magic missile” that's also fine. Etcetera in various combinations. If they want to write plain “magic missile” that can also be fine, depending on the context (often in titles it's not unclear at all), but sometimes isn't (you'll know it when you see it).
Post Scriptum: Clarity's needs can go both ways
Sometimes formatting will actually make a post worse to read, not better. I remember a post that Formatted every instance of a game term, in a post heavy with them, so that it looked like a leopard that had rolled in salt and pepper. It was quite hard to read. Some of those were unnecessary for clarity, and the post's clarity and ease of reading were substantially improved by removing some of the formatting.
The guiding principle is always clarity: is this edit improving the ability for others to understand and agree on what these words mean, so that voting and other site things can proceed accurately and smoothly?