All kegs have a relief valve to protect the keg from failing due to overpressure. Not having one would be dangerous. On some kegs, that valve also has a pull ring allowing you to open the valve to vent the keg. As others have said, there are alternate ways to vent a keg if your relief valves don't allow for it. I use this poppet valve depressor.
I too rarely replace O-rings or seals, but I always keep spares on hand. As noted by others, keg lube helps a lot. I get all that sort of stuff from keg connection. I've never replaced a dip tube.
You really don't have to keep the keg pressurized and/or cold for long term storage. You just don't want any oxygen in there as that can oxidize the beer. After cleaning some empty kegs, I fill one keg completely with sanitizer, close it up, then use low pressure CO2 to force the sanitizer into the next empty keg. That way you are sure the keg is full of CO2, with no oxygen. I leave them like that until ready to fill. A fill time, I attach the above depressor to depressurize the keg, fill thru the liquid side using a disconnect, then remove the disconnect and poppet depressor. The keg now holds only beer and CO2, no oxygen to worry about. I can store, chill, and/or pressurize as desired without worry. A side benefit here is leak detection - the empty keg, left pressurized, should still be pressurized at fill time. If its not, then it is leaking.
That said, it may be more convenient to pressurize a full keg before storage. It takes time for CO2 to dissolve into the beer, up to a couple weeks. Once fully carbonated, the beer should stay that way without needing a CO2 connection, as long it has no leaks. Note that the level of carbonation (volumes) is a function of both pressure and temperature. See a carbonation chart. For example, if you want to serve your beer at 40 degrees F with 2.5 volumes, you would set your serving pressure to about 12 psig. If you are carbonating that beer at 70 F in advance, you want it pressurized to 29 psig for a couple weeks. (The pressure will drop when you chill the keg prior to serving. Similarly, if you carbonate the beer cold, then take it out of the refrigerator, the pressure will rise.)
Welcome to kegging. There is no turning back now.