Based only upon my experience with early V8s you will know you need a valve job when your chain jumps and you bend a bunch of them.
My shop has done probably 300 valve jobs on early V8s. They were almost all caused by chain failures. The only other reason was valve guide failure. The real early motors had seals so hard they wore out the top of the guides and we did many jobs for that reason. Since they changed the seals in the early 80's we have had maybe 3-4 motors with a loose guide.
There is no reason for premptory valve jobs. Just keep your chain well maintained.
Bosch Master, ASE Master, L1
33 years MB technician