While it's possible to press Y+B and even X+Y+B+A simultaneously, it feels kind of wierd to use a single thumb for 4 buttons which has to be combined.

What is the optimal setup? Should I rewire light and medium attacks to LB and RB? Or maybe just stick to the defaults?

Personally, I've been enjoying the most recent (Chrono Phantasma) BB's control set up on the PS3. I've converted them to Xbox 360 controls for you:
A - Drive Attack
B- Strong Attack
X - Weak Attack
Y - Mid Attack
Right Bumper - Taunt
Right Trigger - I set this to my most used two-button combination
Left Bumper - Function 1*
Left Trigger - Function 2*
Click Right Stick - Over Drive/Break Burst - All four face buttons at once
*Functions can be anything from menu options during character select or settings during Training