This is what I do...
I picked up to Behringer ECM8000s. I space them 7" apart, and then set everything up as we would if playing a show in that room (in a line or a u-shape, etc) Then I record the entire practice, bump it to garage band, chop out all the dead space, and pan each track hard left/hard right.
I don't have a proper manikin head, but this works pretty ok. I actually take a tape measure and make sure the capsules are exactly 7", which seems to help. Everything rings through clear enough to record rehearsals.
I wouldn't bother putting mics on individual instruments, though. It's time consuming, and in the end I'm just trying to remember what the fuck happened during jams, etc.
What mics are you using? In this case (I used to record practices by just hanging an sm58 from the ceiling, very crappy) I would consider any dynamic mic to be totally useless. You can get a pair of naiant small omni condensers for about 75 bucks, I believe. They'll do the job.