I've had a gun safe in my basement for 10 or 12 years. I also use a heater rod or "Golden Rod" inside, and check it frequently to make certain it's still working. You can also use the dehumidifying crystals. They change color when no longer effective, and by heating them in your oven, you can restore their function. I've never had any rusting problems, however, I do check my guns very frequently. Kinda paranoid about problems. I also run a dehumidifier in the summer. Blocking the safe up is also a good idea. If you block it up, make sure you have it tight against a wall you can bolt it to.
Bigcountry's right----- you really can't get too protective about rust. It's probably your worst enemy. Check 'em often-----