Hey, Henrik,
TL;DR It doesn't really matter.
Long answer:
IMMS uses song/artist information for two reasons:
1) to recognize remixes of the same song, and treat them as the same song for the purposes of recency demotion
2) to assign default ratings to new songs by a "known" artist
Either way it doesn't trust individual songs as the source or artist data -- it'd be too noisy -- it looks for confirmation in directory name, or other songs in the same directory. That's why compilation albums typically don't get a "known" artist. Not a big deal.
Have fun,
Michael.