I generally commend someone at random, unless someone REALLY stood out. Like, if I do a raid or trial and we almost wipe but a healer survives to lb 3, they're getting my comm. If the party almost wipes but somebody solos the last few % of the boss and gets us a clear, they get my com. If there's a newbie int he party and someone is very helpful explaining things, I'll give htem the comm.

But it's ultimately rare for me that anyone stands out in a good way because when things are running smoothly I tend not to atribute it to one specific person, where as it's easier to pick out someone that's playing poorly like a dps that refuses to aoe etc.