You can use B mode at any time without worry. As others point out, the only caveat is that cruise is disabled in B.
As for brake wear - don't worry too much about it, because the car employs a brake blending strategy. When you press the brake pedal, the car is actually engaging stronger regen...
I disagree with the statement above. The 12V battery is supported during both ready-on and during charging of the main traction battery. If one measures voltage at the 12V aux battery they'll find about 14.4V during both ready-on and charging, but something around 12V when the car is off.