Well yes, it does matter.
Trouble is, there is no "right" answer.
It's a performance thing.  

Assuming you have multiple clients backing up concurrently to your disk
pool, TSM will start as many I/Os as there are clients sending data, up to
the number of  TSM "volumes" in your disk pool.

If you have more "volumes", then you get more I/O's in flight concurrently.
That's a good thing and will improve performance, until you get "too many"
in flight, then the effect of yanking the heads around degrades performance.

It's even harder to figure out what is optimal in a RAID situation, since
you don't have a 1-to-1 correspondendence between your TSM "volumes" and
physical disks.  And most RAID setups have some cache that acts as a buffer,
and that helps improve performance but further disassociates the number of
concurrent writes from the number of physical disks. 

So think about it this way:  How many concurrent WRITES do you want to occur
in that RAID pool?  Pick a number, and create that many TSM "volumes".