Don't see why people would advise against it - I tend to advise people to do that, for the same reason you noted! It's a lot easier to service the manifold gasket on the bench than on the engine - by a couple orders of magnitude...
Granted, I'm a fairly large individual, and waving around a two-foot long iron casting isn't that big a deal for me, but if you're going to do the job and you don't have the power to do it yourself, get someone who can help you who can follow instructions immediately and accurately. Two extra hands helps - but they've got to be hooked to a single brain order to work...
5-90