Load up 6 of the cards with the stock bios and test just a few cards with the stock bios. Start adding cards, see where it breaks. I def prefer adding 1-2 cards at a time rather than a full rig at once.
Yep I do one at a time, flash bios, test hash, remove card and start with next one. It takes longer to set up a rig, however the time saved troubleshooting one card/riser is a blessing.