So in regards to composition, is centering bad in concert pics?
I don't know about concert pics, specifically, but in general, yes. I should let you know that I haven't much experience with portraits or live events. I can only tell you what I've picked up from reading and posting on this forum. Generally, off centered subjects look more interesting to the eye than centered. However, in portraits, centered subjected seem to be ok. Especially with tight crops. Someone with concert experience want to chime in here?
You mentioned dead space. How do you determine dead space between space that works for the pic?
Is there anything interesting going on there? Would the picture be better without it? In the case of your 3rd picture, you can get rid of alot of the photo under his hands because it's just jumbled wires and some mic stands. You can also crop out the smaller window to the right because it's a different color and destracting. The one on the left pretty much has to stay because he's in front of it.
Where and why I cropped it:
How it looks after cropping:
You could do the same on #2 and #4 to remove the glowing band member and the floating detached arm.
Another question to ask when looking at how images might be cropped is "Where is the subject looking?". If the subject is looking or motioning to the left, give them room to do so. So in #2, I would crop just enough of the right side to remove the glowing band member, and most of the left side (just past the guy in the background). That way he has somewhere to look and there's no obvious distraction in the photo.