Bob, for my sanders I always leave them tensioned, I've got three belt sanders and a couple of handheld power sanders that use a narrow 1/2" belt plus my orbital sander converts to a belt sander and I've never had a problem with any of the belts. None of the belts have ever let go or separated, and there's no need to worry about the belts taking the shape of the rollers while sitting there. The moment you turn on the sander it will rotate the belt fast enough to let it work properly. If I were to release the tension on the sanders, I'd be forever forgetting to put the belt back under tension before hitting the on switch. The only power tool that I release the tension on after using it, is the Band saw, and I often forget to reapply the tension before using the saw. That is always a bit exciting to say the least, but luckily, I've never ruined any blades or did any damage to the saw.<br />Len