I would use a mat of some sort, mainly because the glass in a 5 gallon tank is so thin. A couple ideas that have worked great for me in the past.
Go to your local hardware or tool store (Sears, Lowes) and in the tool box area buy a tool box liner/mat. They are 1/16" to 1/8" thick usually black and easy to cut to size.
Or you could go to your local craft store (Michael's, Joann Fabrics) and they usually sell some thin foam sheets, the type that kids cut shapes out of for craft projects. These come in different colors and thicknesses but you'd probably be fine with the one that is just under 1/8" thick. I like the black color the best as it blends in better.
To cut these to size I usually put the mat on a piece of cardboard, place the tank on top and hold down tightly as I take a very sharp razer knife around the base of the tank cutting or at least scoring the pad.
I agree the 'theredben' this is very cheap insurance.