Full length or waist up?

When taking group shots of a few ppl, is it better to take full length body shots, or shoot from the waist up. Or is it one of those "Totllay subjective" type things?

Also, is it better to take standing or crouched?

Generally, if you can arrange a group of people so their heads are not at the same height the results will be better...avoid the row of heads look. I generally don't think it's important for my photos to prove that all the people were wearing pants

I've struggled with this problem myself (and still do at times).
I recently had to shoot some equestrian teams comprised of Jr. High girls and their coaches (mostly women). Each team had different number of subjects and obviously different shapes and sizes of individuals. For the coaches (adults) I use the 'just above the waist' view whenever possible. Reasons: sometimes a GREAT variation in size of the ladies. If it is really noticeable, I put the larger of the two in the back and separate them to diminish the differential. If its not too obvious you can turn them to the old 3/4 profile shot. Seldom do they have uniforms/costumes so full length isn't necessary. A lot of the women I shoot are (diplomatically) mature in build..... They often appreciate head/shoulder image rather than the less flattering view.
With groups requiring two or more rows (because of background and/or lens restrictions) you sometimes have to break up the natural tendancy to stand next to your 'buddy'. All in all I've never been in favor of the 'baseball team' look. I encourage the idea of a collage - each members head shot vignetted against an appropriate background. You can sometimes even impose names under them.

It's a totally subjective type thing.

I've always tried to do the "chin to forehead" measurement rule, but sometimes you've got to just make it work so that it looks good to you behind the lens. Groups can be challenging at time, especially at different heights. I've also used boxes and phone books of various thicknesses to get people "just where they need to be" in order to get a shot. So yeah, you can stagger them so that they are arranged from chin to forehead, at the right heights, etc. if that works for you. Otherwise, just have them arrange themselves and watch the fun... I've gotten a lot of good pics that way! I've also tried alternate male / female and even clothing colors / types, so whatever works for you and the client is a go. I don't think there's really any "rule" per say, but looking at a few good galleries and some posing books will definately give you some ideas.