Yes the lockers are safe. No one steals from you if thats what your asking. But you can't store to much in the lockers, keep the items you really like. Always keep all the ammo you find (you never know when you may need it) If you are a grenade person keep those but i usually sell them. lets see what else.
oh yeah! once you get a vehical you can keep just about everything in there. I don't think there is a weight limit.