My recommendation is to always release the belt tension, especially at the end of the shop day. Why? It's not about the spring(s), it's about the contact wheels. Experience has taught me that IF you leave belt tension on the machine, it will cause flat spots on the contact wheels. The smaller diameter the wheel, the more prevalent and intense the flat spots will be. When it first happened to me, I could not figure out where that "ka-thump", "ka-thump" was coming from, but finally got down to removing the wheels and testing them by rolling them on a granite surface plate. The wheels stopped on the flat spots......and I had to replace the wheels.