Well I think it's just going to be personal prefrence on your part.
Back in 2005 I noticed coolant leaking out of the head gasket. It wasn't getting in the oil just leaking out. I also had oil leaking from somewhere that I couldn't tell where it was coming from. I did oil samples every oil change (12000 miles) and it never showed any coolant in the oil. I ran the truck for quite a while.
Then in December of 2005 I wanted to bring it in to Detroit to have the bearing checked, oil pan gasket replaced, and then the service done. The mechanic did an inspection and noted several oil leaks. I didn't like oil leaks so I wanted them to find out where they were coming from. They said they would put dye in the oil and next time in we could find them. I didn't know when I was going to be back so the suggested a dyno.
So they put the truck on the dyno and found out the oil was coming from the head gasket. Since it was leaking both oil and coolant I said just replace it. When they pulled the head they found out that my valves were sunk in the seats way past spec's. The said I could either run it or in frame it. I decided that since they already did the bottom end and they had the motor half way torn down to just have the in frame done. I had the money in the bank so why not.
I don't know if anything would ever show how the valves are besides just going in there. I didn't want to take a chance and sink a valve. I am glad I did the head gasket because if I didn't who knows what would have happened.
If I were in your shoes I would do it, you definitely got the life out of that motor. My in frame was done at 800,000 on a Detroit series 60 12.7L