When sanding 1/16" sheeting, or trying to true up some ribs, I can imagine that a twist of 1/32" could cause problems or even gouging.

I can't say that I've purposely checked my sanders, but I've had them sitting on the buildboard and used them for straight edges and guides before, and I've never noticed as much as the slightest twist or unevenness in any of the 10 or so bars I have. They seem to lay flat enough that the biggest uneveness I've see has been the result of either balsa dust under the paper or lumps in the adheisive back.

Scott