Indeed....never run a carbide blade backwards.......It's getting harder to find non-carbide blades. It's important never to run a carbide blade backwards. All the teeth will fly off and become projectiles. I have a stack of blades I picked up from an old guy. If I'm doing something that may hurt the blade, I grab from that pile.
I've used the same 7" plywood blade backwards for cutting miles and miles of aluminum siding/soffit.
Pipe is likely harder on the teeth.