It's an old thread, but I'll throw in my 2 cents anyway...

As several have already discussed, you don't want the derailleur pulley hitting the largest cog on the cassette, so that is an obvious, useful adjustment. In my experience on my mountain bikes, shifting will hesitate or stall completely in 1 or 2 of the middle gears if the B-tension is adjusted with the derailleur pulley too far away from the cassette. The chain will hesitate a few seconds or stall until I shift ahead another gear, then it will suddenly jump 2 gears, effectively catching up with the shifter position. When I adjust the B-tension where the derailleur pulley is as close as possible to the largest cog without hitting it during the shift to that cog, then shifting becomes precise again without hesitations or stalls. This is the B-tension's purpose. It is NOT there for adjusting the "tension" in the chain. That is accomplished with the correct chain length (removing appropriate number of links) and the spring pulling the "swing arm" of the derailleur.