Barry, You are right on as far as not hurting 'heads-up' hounds by starting them on rabbits.
I believe a hound is born with a natural inclination to be 'heads up' or 'heads down'. How you start and train them can push them in a direction you hope they'll go.
Putting too much pressure on young, competitive hounds can result in road-running, throw-away hounds that are useless without a pack to run it for them. On the other hand, if you hope to compete natural track runners, you have to put some heat on them to see if they can learn to get their heads up.
In pens, I prefer to start my pups on foxes -- cross foxes work really well. With the blood lines I fool with, it's much more important for them to learn how to run a track properly. For the most part, 'heads up' came in the pedigree.
One other point -- as you know, Sheridan and others used to use snowshoe rabbits a lot to train young hounds.
That was when we had a lot of rabbits, foxes and coyotes and relatively few deer. For those wanting straight fox or wolf hounds, starting them on rabbits can work against that.
Some hounds that are started on rabbits adjust quickly to the desired game. Others have a real inclination to be 'trashy'. Any foxhound that runs a rabbit is a prime candidate to run a deer. Depends on what you want.