I'm using Magento 1.6.2.0- and I can do this by creating a shopping cart rule for each tier of quantities. see this page http://www.magentocommerce.com/knowledge-base/entry/what-are-shopping-cart-price-rules-and-how-do-i-use-them
What I would do is create one rule for a percentage off of 5 or more, then an additional rule for an additional 5 percent off of 10 or more....and keep creating a rule for each tier