Yes... All Mario, Final Fantasy, Metroid, and Castlevania hacks use the original game's boxart (as well as any other hacked game for any other system).
I think there's only one time I actually used a Rom Hack generic boxart, and that was for a prototype or something that got hacked and it didn't have an original boxart.
I think this gets a little confusing for somebody who just dumps all of the roms into one folder, but this is why I provide datfiles broken down by type so people can break them up properly. There are actually so many hacks for NES and SNES that I've broken them down by game if there were more than 4 hacks for a specific title. (Think Metroids, Marios, Zeldas, Final Fantasy's, Castlevania's). It looks really sharp when you have HQ boxart for all of them as you scroll through the hacks.
EDIT: Just going through a few more NES hacks now and I came up with an example of when I'd use a Rom Hack image. We had a few hacks for Earthbound Zero, which everyone knows is a prototype/unreleased game. Since there was no boxart originally and we used the "Prototype" boxart for that game before, I used the Rom Hack boxart for the 2 or 3 hacks we have for it now.