I clean after every use. propane is a fairly dirty gas so when you fire it, all the garbage that's left over sticks into the silicone oil you use to lube it. After a game, it's most important as you've introduced dust and junk from the game field into the lubricating oils.
I was diagnosing some issues with hopup and put about 12-15 magazines through my pistol, and by the end of it, the lube on the slide was bone dry, or contaminated with propane. So I had to lube more to continue else the slide would not cycle the gun properly. And this was indoors with no dust/dirt in the air... imagine the crap that gets into it from a game field.
That's probably the part that needs the most attention, on top of lubing the mags as green gas comes with silicon lube mixed in, where propane does not (already mentioned).
Make sure you use a non propelled silicone oil. Anything that comes out of an aerosol can likely has petroleum distillates in it, which can eat/damage rubber and plastic. Hobby shops carry some pretty decent stuff for RC cars.