For the most part I wire the bec in to the power plug on the esc that connects to the battery. On one occasion I powered the beck with its own battery.
I prefer not to touch the balance leads.
Seeing as I have to solder a battery connector to my esc anyway it is no big hassle to attach the beck there.
Always check the input voltage rating on the bec before doing this as some cheapy becs cannot handle higher input current.