hmmm.. i've read few posts (not only in this forum) about ppl forgotten to turn on their valves.
i don't know what they are thinking there.. but i thought when u r attaching your reg to the tank, u r supposed to turn it on and check what is the pressure reading? and after that, just leave it as it is?
and probably, after u put on your bcd, u depress the purge button once or twice to see whether got air or not... it's such a basic things, but i heard stories of ppl forgetting to turn on their valves... :-? ... it's not even the whole BWRAF yet :
and that said... i guess it's probably good that u attach your reg yourself, rather than other ppl doing it for you 
sorry john.. not directing at u.. but it's general.... peace 
SKK .. its all depand on where about you dive..
normally in a dive resort, before dive briefing, the DM will normally ask you to check
your gear after they assemble it for the divers. This including you yourself check the air from the secondstages and the SPG then turn off the air. After checking they will ask you to carry to the speed boat. Upon arrival at the dive site then turn on the valve back.
Its quite difficult to do the BWRAF on a boat with 6-10 divers onboard. but a good ethical dive centre will will emphasis this process with thier fun divers or students.
It will different case if you are on LOB. too many ppl jumping at the same time and limited space. so your buddy will be doing it for you. So ensure you gear is close to buddy's gear.
As what others mentioned earlier, BWRAF on you first then force yourself to do on your buddy.You need to know whether your buddy equipment works alrights bcoz if you low or out of air, you will be chasing your buddy for alternate air source or mayby buddy breathing stationary where you will be the receiver.
As I said different places may have differnt perspective on this issue.