There are a few threads I've read here and there that suggest even smaller (~6") plecos have caused "scratches" on acrylic tanks. There are also threads of owners of (small) plecos stating that they have no issues with their plecos' teeth scratching the acrylic. I was also curious as well since I recently got a new acrylic tank and I've not concluded either way.
I have 3.5-4.5" blue phantom and gold nugget, and I've not seen evidence of scratching, yet. The reality is unless they are putting some major scratches, I don't know if I'd change anything.