We don't need to enforce a style policy about exactly how people should use bold and italics. It is okay for different posts to use different styles. There is no one proper way, nor probably even just a few. Trying to mold authors to one style would make it artificially hard for people to contribute to the site.
Having a policy about bold or italics that makes authors feel obligated to go and look something up about proper usage in the middle of writing is exactly the opposite of what we should be doing.
Why do people edit in (usually pointless) bold and italics to questions? serves the function of addressing the situation where bold and italics are used in a way that actually causes problems. In terms of rulemaking, I don't think we should be going beyond that.
And as Oli says, we do have the style guide as well, where detailed suggestions and recommendations can be set forth. (But I recommend only editing the existing answer there if there are suggestions that seem as important as what's already there. Right now the answer there, for the most part, offers just a handful of particularly important guidelines. It is neither overwhelming nor capricious, and I think we should keep it that way.)
I'm not (just) objecting to this question; I'm (also) answering it:
- Acceptable uses of bold and italics are any use that doesn't actually cause problems. If it doesn't make a post harder to read, we don't need to force (or otherwise push) people to use a different style.
- Unacceptable uses--which is perhaps an overly strong term, but I mean bad uses that are important to avoid and to fix by editing--are those that actively harm a post's readability.
One might object that those aren't really rules at all, but instead common sense that has to be applied thoughtfully to each individual post. And that is the point. Making sense, in practice, for actual human beings, is considerably more important than satisfying abstract rules for writing.
In case that's deemed inadequately concrete, here are three very general guidelines, which do not need to be enforced with an iron hand, but which may be useful to both authors and editors. Note that I am not even saying these very general suggestions should be considered a site policy--not even if virtually everyone happens to agree with them.
Whenever bold or italics don't really achieve anything, it's best not to use them.
One of the most common ways bold or italics can actively cause problems is when they're overused, to highlight either (a) too much text, or (b) text that doesn't need highlighting at all.
It's usually best to apply bold and italics in the same general pattern throughout a single post (though there is generally no need or benefit to deliberately have different answers to the same question, or different posts by the same author, do things the same way as each another).
For example, if package names embedded in prose are italicized in some places in a post but formatted as
code in other places in the same post, and there is no contextual reason for the difference, that is likely to be confusing.