Yes it sounds like the system is de-pressurized. You could, for grins, also depress the high side valve to be certain. If you have refrigerant you should probably have it recovered so not to allow it to go to atmosphere. Also anytime you are working on a pressurized/potentially pressurized system wearing gloves and safety glasses are a must!
Let's go back to why you thought you needed to add refrigerant. Assuming it went from blowing cold air to blowing hot air you should think about doing a little more diagnosing to get to the root of the problem. It would be, and to disclose I'm far far from being an A/C tech, relatively rare for a valve to fail apart of day-to-day use of the A/C. Without a leak source of some kind A/C systems are completely closed systems and do not need to be topped off with refrigerant.
I would suspect you have a leak elsewhere in the system. At the very least you might want to assume you have a leak/failure in the system. You can start by checking the simple things: A/C fuse and also look for any tell-tale line/hose punctures/failure points where easily visible. The A/C clutch should be on your initial list of items to check too (outlined how to in the FSM...can post if you need it).
Regarding my recent foray into all things A/C work on my '99 I started with a $45 inspection investment at the local A/C specialty shop. They quickly ID'd the problem and then I took over doing all the work, vacuuming and charging the system.
If you know there was UV leak detection dye added to your A/C system you can use a UV light and yellow colored glasses (~$20 from most auto parts stores) to try to find the source of the leak (assuming its not a bad compressor).
I'm not sure if the high or low side valves perform as a safety release or not. If so then it sounds like maybe, just maybe, when you added more refrigerant you might have exceeded the capacity of the system...dunno, just a guess. Then assuming you don't have a leak/line/hose failure then maybe this would point to a A/C compressor/clutch failure.
