I tend to agree with Kevin, Gerardo, and Phi concerning movement. In fact, Ueshiba stated that to place the feet in six directional posture. Up/down is included in that. Speaking of ... watch video of Ueshiba. Slow it down. Notice how he moves. Not a lot of sliding going on. Lot of stepping, though.
If you don't have any up at all then it's going to be very difficult to move on any normal surface, that's just the physics of friction. OTOH, there are ways of moving with an up component that don't have the the body actually moving upward and bobbing up and down.
This should be a no-brainer for anyone who trains outside on a regular basis.
More to the topic - heel or toe or bubbling well - what mostly matters is the weighting, IMO. Also, I'd say that turning badly accounts for most (all?) of the knee problems that are so common among Aikido folks.