You could pack every inch like a Tetris master, but that makes retrieving items very difficult. A little extra space can give you room for shelving units and a path through the middle of your unit for easy access to most of your belongings.
It’s also a good idea to use some space to place something on the ground like wooden or plastic pallets to ensure your furniture, appliances, and especially boxes aren’t resting directry, on the ground where they’re vulnerable to spills. Obviusly, you don’t want to pay for space you won’t use, but Life Storage’s Size Estimator should give you a good idea how much space you will need.
