So I am in the process of refreshing my '90 5.3l. I know that the recipe calls for valve seals only on intake valves, but since I’m at it anyway, and I have 24 seals, is there any valid reason exhaust valves could not get seals installed as well?

 

Lots of different opinions. The main reason not to is so that the valve stem gets a smidgeon of lube. Remember it runs VERY hot, and there are stories - true or not I have no idea - of exhaust valves with stem seals failing in short order.
My V12 engine and thousands of others have done multiple hundreds of thousands of miles without the seals, so why change the OEM design?