I follow a very simple rule: if the decal represents a sticker or vinyl graphic on the 1:1, I don't clear over it. For example, many 60s-70s muscle cars had stripes and graphics that were diecut vinyl, like the "hockey stick" on some MOPARS and the "strobe" pattern on AAR 'cudas, and were applied over the finished paint, and were noticeably less shiny than the paint. If you want your model to look "real," you wouldn't apply clear over those decals and bury them under a mile of clear topcoat, because that's NOT how it was done on the original.
However, if a decal represents what would be painted on the real car, like flames on a hot rod for example, then it would make sense to clear over the decal... if for no other reason than to hide the decal edge.
And finally... the idea that decals need a clearcoat to "protect" them is nonsense. I've been buildijng models for a long time, and there has NEVER EVER been a time where any decal has ever popped off the surface because it wasn't "protected" by clear. If you want to spray clear over decals, that's your call... but there is no need to "protect" decals with clearcoat, they're not going anywhere.