2002 E320. The AC makes a hissing sound once in a while. I took it into the shop and they said it blows cold and everything is fine. I told them it might me freeon. They said there is nothing wrong and it blows cold. I think I might have a leak? Or could it really need freeon?

There is only one way to know for sure if you are low on refrigerant (but technically, what you use is called Suva rather than Freon), and that is to reclaim and weigh what is in there now.

Has the system been serviced in the past? It will naturally leak a bit, so if it is the original charge it is likely to be low. Low refrigerant level causes insufficient oil return to the compressor before it causes the system to start to blow warm. Insufficient oil return causes compressor damage.

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

The problem is that while the refrigerant does return, the oil doesn't do so well in an undercharged system.

http://www.airsept.com/Articles/RecycleGuard/Motor0403.pdf
They're writing about a level of refrigerant loss that will NOT result in customer complaints, until the compressor eventually fails.