Personally, I'd generally advise users to leave profile mismatch checked, for 2 reasons,
1: As evidenced by it's, embedded profile, an RGB file may be in, say, a scanner colour space (which is quite possibly non linear), thus it should be converted to a working colour space (those are linear) before editing since edits in a linear colour space are more predictable.
By @NB, colourmanagement net
There is a much simpler way:
It's not that the mismatch warning does any harm in itself. It's more that it misleads inexperienced users into thinking it's actually important, and that the document profile should match the working space. Every time that warning comes up, this misconception is reinforced.
So to be absolutely clear: the embedded profile does not need to match the working space.
Now CMYK comes up, which wasn't originally mentioned. All explained above, but it needs to be particularly emphasized that there are scenarios where you don't want to convert at all. It mainly involves overprinting black plate, 0-0-0-K. This applies to e.g. text, but also things like comic book illustration and so on. If you convert, those K-only blacks will be turned into four color blacks, and that can be a minor disaster in some situations.
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