Mine's in my closet, so there's not an issue with moisture.
If you do, make sure you put in a heater bar or better yet, a dehumidifier (not dessicant packs, they get expensive)
Like this: Amazon.com - Eva-dry Renewable E-333 Wireless Mini Dehumidifer
Whatever you do, make sure you bolt it down to the slab. Especially if it's in the garage and you're everytime you open the garage door, you're advertising to your neighbors "hey, look here... I've got some stuff worth stealing inside this here safe".
Mine is in the garage, we had problems bringing it in the door, a buddy kind of dumped it on my chest, so I said "that's it!", and set it in the garage. Hurt like heck when it landed on my chest, nice bruise line across my torso. Seems to have a decent seal and I have not had rust issues inside. The 'spinner' handle to lock/unlock has some rust on it but IMO it is cheap plating so it doesn't surprise me. Safe itself, and the contents have no rust issues.
Starting out I wanted it in my closet, but I'm happy with it in the garage. Probably going to move it in a few weeks when I re-arrange. Will move to another spot in the garage, not indoor.