Yes, but not really, since you can always specify the chroots a specific package should be built in.
However, this is what I do for my ~50 nightly builds packages in COPR: clean, condition-free spec files in official fedora branches, and a "combined" spec (with conditionals accounting for differences between fedora branches, if any) for nightly COPR builds - so the maintenance overhead is very low, and the chance für bugs is small, since the "combining" is trivial (copy-paste, usually) - but the canonical versions are if course always the conditional-free specs from fedora. I wouldn't want to use the "combined" spec there.
Well, the good thing is, nobody cares (or can care, with the amount of packages built there) about Packaging Guidelines in COPR, so that point is moot anyway
Fabio