you can plug the motherboard into the psu and switch it on outside the case, just to check that it receives power etc...and yes, don't do that if on plastic. but i checked, i did rest the mobo on the bag, on the mobo box..offeres a little protection to the back of the mobo when pushing in the ram etc. the anti static bag the mobo comes in is non conductive itself, so don't see much of a problem. as said, I plug psu in to wall and touch it every so often which grounds yourself(again, psu doesn't have to be on)..that and I don't build while wearing trainers on a carpet..also if you next to a radiator or something, just touch radiator also..your pipework in the house will be grounded also.sry i dont get it. What do you mean by powering up the motherboard on the bag or on the motherboard's box? dont i just assemble all, put in case and then power it up?
I tested all my fans etc with the psu before putting them all in the case..didn't with the mobo though..put that in case and screwed in 4 corners then connected it up to test iirc. only when everything worked did i put the psu in the case and then start all the cable management. Be soul destroying to do all the cablemanagement and when you turn it on, find out you have a dead mobo (unlikely though that is)