Great question "Don't know if the shop guy tested it on pos or neg grd. So I will polarize it before I start the engine. Now, once a generator is polarized, does it stay that way, say if I had to disconnect the battery, or would I have to polarize it again after reconnecting the battery?"
Its more common for a shop to have it polarized at NEG then POS ground but it never hurts to be safe and polarize before start up ESPECIALLY IF YOU ARE POS GROUND.
Just sitting idle for a relatively long period or simply disconnecting the battery shouldn't cause the gennys soft iron field poles to loose their residual magnetism and it doesn't need re polarized. I think if it was disassembled with the armature removed it may loose polarity sooner.