If you have the Uniform Hazmat Alliance permit, as most chemical tank companies do, you don't have to go into the scales on Ohio.
I been hauling Haz and been in the line at the westbound Arizona scale on I 10 and seen many placarded trucks go by and never saw anyone chase them down.
In Arkansas, I have seen the DOT chase down placarded trucks that don't go in. Friend of mine got pulled over west of the Hope scale. Said fine was $125 and the cop did give him a PrePass pamphlet.
Ohio, Arkansas and Arizona are mandatory that I’m aware of. They will have signs if they require it. Many major cities have bypass’s for hazmat. Make sure you follow them. Maryland has special lane restrictions for hazmat as well.