Not only is it usually easier in my opinion to do that job with the cross-member removed, but along with what atilla mentioned, it's also easier to get the proper torque on the pan bolts when you've got a straight shot at them (not using a universal joint which can sometimes result in over-tightening due to universal socket binding).
Clean the trans bolt threads first, get everything in place, apply some thread-locker to each bolt, then torque appropriately. Less chance of over-tightening/stripping pan bolt threads.