You can replicate load points run after run to check and test fuelling / ignition at each rpm point. You STILL need to check the engine once in the car as well, but this will be minor tweaking by comparison.

In my own mind, I would NEVER dare max out a car in fifth that had only been mapped in fourth. I would want piece of mind to know that it had been checked in it's worse case scenario, as shit happens at 7500rpm in fifth .