I've done it both ways. If it is JUST a timing cover gasket that's leaking, I would probably do that in the car and pull the tranny separate. If more work needs to be done to the engine, I would pull them together.
This Walker434... If you need a dedicated transmission jack, Harbor Freight, does an adjustable one ( up and down travel about 15 inches, adjustable riggers for different trans pans, a safety belt already there... ) Runs about 50-75 bucks. I consider it ,essential.