I've seen a lot of AC units hiss from time to time; usually when you kill the engine, but many times when the compressor cycled on/off while modulating the temp in the car. I always assumed it was normal noise from the expansion valve; but come to think of it, I don't think my MB has ever hissed. But, if the shop checked the pressures and they were proper, and it cools well, I guess we could assume the freon is sufficient. Since the acumulator holds the excess freon, and is on the high side, and the compressor gets it's freon/oil supply from the low side, I would again assume that the expansion valve is flowing all the freon/oil the system is requesting, and all should be fine.
Doing a lot of logical "assuming" though, I'm certainly not an AC guru.

DG