My folks' AG pool has a gorilla pad under the liner, and it makes the floor of the pool soft to walk on and they have no wrinkles or bumps on the pool floor. My pool just has a sand subfloor with no pad, and over the past several years it has some wrinkles and lots of bumps/dips. Whenever we replace the liner I definitely will put one in. I guess all this to say you might as well do it right the first time and get padding or foam. I don't know that there are any negatives to having one.