Don't run an octane tune that has a higher value than the fuel you are using. So don't run 87 fuel on a 91 tune. Octane values vary by area. It won't hurt to run a higher octane fuel than the tune value.
There is an extensive Predator document library here that may be helpful:
It was initially created with the 5.7L & 6.1L LX cars in mind but is still relevant for the trucks.