There's no concrete number of maximum utilization. Different issues get less comfortable at different points.
If you want to bt some money from a card at 0% to a deposit account and just make your 5-6% on it, this is perfectly safe, but using a large portion of your credit lines will reduce your score. Although remember that a score is just a means to accomplish other things you want, not the ultimate goal in itself.
On misc. thoughts, don't forget to leave room for bt transfer fees when thinking how much to transfer. Don't forget to make on time payments (not only will this have negative impact on your score, but I'll leave to you to figure out what happens when you have say 50k or more at 30% interest rate because you were late on a payment).
There's plenty of useful reading on this subject on fatwallet finance forum. Just search for app-o-ramas.