[Akiyoshi]

Remember that Yuuno can also fly totally unassisted.

Probably Nanoha is too accustomed to fly using Raising Heart as it require keeping various things in check around her(stability, direction, speed, etc...) whole shooting and using "floater" are much more easier spells to cast and thus she's able to do it without RH's help, we also already seen her using Round Shield and Divine Shooter unassisted way back in A's.