I tried to order some plywood from Aircraftspruce today. They were out of stock on every 3/32" sheet I looked at. I ended up ordering from Balsa USA at 2 x the price/sq. ft. It is 5 ply instead of 3 ply. I know it is technically a more stable and rigid product, but is it worth double the money?
Yes, it is WHEN your application needs it. I cut center wing spars for, well the plane in the picture. It has a slight amount of dihedral and a straight piece of wood wouldn't work, so I cut it out of 5 ply and slide it in. Very strong. Next one will be straight and I'll work a spar tube into it. I KNEW it didn't need dihedral but I followed the planes. I like how it flies for doing mindless touch n gos at the price of a gallon of gas, some 2 stroke oil and about half a summer of flying time on that. But if you're doing non-structural things like servo mounts or light electric motor firewalls, it's probably overkill. Gotta use what you can get though. There might be plastic, cutting board material that could take the place of the 3 ply OR 5 ply for non structural pieces.
You don't build carbon fiber Egg Cartons ( you could) and Boing doesn't foam wrap their wings, thats what Carbon Fiber is for. That's a 3 ply to 5 ply comparison.