I pretty much keep my nails cut off either way, for piano or guitar. They used to get caught in the piano keys, but then I couldn't fret a guitar with long nails (I leave the ones on my right hand long because a) I'm right handed and tend to do a poor job of nail-trimming with the left hand, and b) I use a flat-pick so I'm not playing with the nails, anyway).

I'm going to learn a bit of fingerpicking one of these days (years . . . decades . . . ), I swear. Right now I'm always the back-up guitarist for a bunch of other instruments, and we don't have a bass, so the aim is for sound volume and not intonation. Flat pick and a lot of pounding on bass strings. It's not pretty but, by God, I'll keep everyone on the same beat.