STP root switch question

Folks: I have a fairly simple network, with a potential for growth and hence the need for proper STP configuratoin. I'm trying to understand the worse case scenario for a misconfigured "root" switch. I understand that I can force the root switch to be the core switch by changing the priority and also understand the use root gaurd protection and other techniques.

Here is what my network looks like:

Switch (M) Core switch and also desired STP root, VLAN 20,30,40 ) / | \\ switch A(VLAN 20) switchB (VLAN 20) switch C (VLAN 30)

Now it turns out that becuase of lower MAC ID, switch A become my root for VLAN 20.

Here is what my STP diagram looks like for VLAN 20.

Switch A (root) DP | RP Switch M (Core) DP | RP Switch B

Questions:

1. Things seem to be working with switch A as the root switch. Where should I look for spanning tree misconfiguration/bottlenecks?
2. What is the worse case scenario with having switch A as the STP root?

-JC