I use a jack for all the reasons posted. Bottom line its safer and easier to get lined up without hurting anything. My JC Whitney transmission jack has worked fine.

I just used alignment studs for a heavy iron transmission on a '47 Power Wagon per factory service manual guidelines. They really helped. Next time I do any transmission I'll use the studs. I made them from bolts... starting with the longest bolt that would leave clearance I cut the head off, tapered the end and ground small flats to fit a box wrench in case it was needed for removal.