yea, you need curves with any IC motor. I tune with my Gov one though, I have tuned with it off and really didn't see any benefit. guys do things differently, as long as the end result is the same, that's all that matters.
you do have a "V" curve setup in your radio in IU1 and IU2, right? meaning curve looks similar to this - 100, 80, 70, 80, 100......that is a example though, the numbers mean nothing, just representation. sounds like when you go inverted and you give negative collective/throttle your going to an idle instead of back up in the rpm range.
I just recently started flying nitro, but been flying gassers for a long time. gasser curves are much harder to get since they don't have a liner throttle like a nitro. but your normal curve will start with your idle %, once you get in hover, you flip into IU1 which is a v curve, as your IU2 is too.