The o-rings are cheap, it wouldn't hurt to do them while the valve cover is up...

 

Well, you could. But really all they test is flow rates. The issue is, especially with "sticky" 6.0 injectors is that it seems as the motor warms up, they clear up and run fine, so they're kind of hard to trace down. Cylinder contribution tests are about the best way, just monitor it like crazy and watch it as soon as you fire the truck up. Even then they're hard to catch.