I'm looking at creating a 'rehearsal' piano part for a SATB choir piece that has frequent voice-crossing. In one sense, I'm wondering why someone is asking for this, as the choir part itself is fairly easy to read, unless the pianist has trouble with the 8vb treble clef. But a piano reduction can't include lyrics, and the voice crossing can result in some lines written for piano that seem kind of messy, unless I un-cross voices - which then may make it unclear how a pianist helping rehearsal can play back each voice's part.

What are some guidelines for creating these that can help me whittle away at unnecessary things to include and generally make the part useful?

I don't understand the problem, and I'm not being a smart @ss. I just may have missed something.

But otherwise, you just put all he voices exactly where they are sung, on a piano staff. If you want to include lyrics, put them below the bass clef.

Is this for the choir to rehearse their parts, or the director to rehearse the choir?

I've done both, but I do them a little differently.

For the choir, I write the parts out in notation with each voice on a separate staff. Then I save it four times, each time with one voice set to loud and the other three set to soft. I email it out as MIDI, because that uses small files and can easily email. Most people have success with any media player. If I need to include the accompaniment that's much trickier to write, so I simplify way down and keep it soft.

Occasionally I run rehearsal and my piano skills are not even close to being able to lead that way. So I do pretty much the same thing, write it in notation, but now I bring a laptop running the notation program and a set of speakers, and I spend a lot of time planning where I'll do repeats and loop sections.