Having made quite a few frameless ones now for males and females, as long as it doesn't bounce on your hip tops or top of your arse (cheeks region) as you walk seems to be the key to a comfortable ride WonderB.
This is something that a lot of people (as do I) seem to agree on.
So yes, an an overly long Rucki- can certainly be a problem.
If it's made to your back length, as FatRatty said, the shoulder strap adjustment should allow fine tuning and optimum comfort.
I'd say go with what the manufacturer of the rucki suggests once you tell them your back length.