look at your topology, is the blocking in the correct place.. Make your decision based up on where you want your blocking to be.. Remember this can be done per Vlan. you can have diffrent vlans take diffret paths depending on what switch you designate as root. if you have a large network, i suggest seeting up a root seccondary as well.
If you dont make a choice, the mac address will decide it for you.
have Fun.