If it's not a really long tank like 6' I would only put shims under the end that you need to lift. My floors here are really screwy too, and everything is shimmed. I only shim the ends, on my 180 I did shim the middle a bit as well, just to keep the stand from sagging. As far as I've ever heard with a glass tank it's only important to make sure the corners are properly supported and level, no twisting of the tank. It's not necessary to support the whole length of the tank.