Mulching is not necessary, but it won't hurt. Mulching will have zero effect on spread.

I agree with Kudzu. Just don't create a huge mound of mulch up against the plant. Add a couple of inches, but keep it pulled back a little.

Thanks for the responses. I'll see how the weather goes. It's been a little hot lately and I just want to be sure the new bamboo has enough moisture to work with.

Well, if it's hot, mulch will help with moisture retention. Just make sure you water regularly. When you have bamboo that is new and not fully established, it's helpful to monitor moisture levels with one of those cheap ($10) moisture meters that have two long probes you stick in the ground.