Does my basement or garage need a dehumidifier?

If you have no standing water and no musty smell or mold, I wouldn't worry about it too much. If you do truly get the "white foam" when it rains a ton, then you could keep it around in case it gets worse. Otherwise, I found that if you have your gutters taking water away from the house and have the ground graded around the outside to push water away and not toward the foundation, you should be fine in most areas of the country without one.