it's hard to hold lower than 7-8% for a long time, and not really good for you. i've been as low as 3% (actually a little lower, the guy said it was the lowest he'd seen) for periods and it's easy to overtrain, get injured and sick, and under perform.
if it's part of an overall periodized program that's a different story. you may peak for a certain period of time or event, but it's impossible to hold your best form all year.
my knowledge of this comes from the realm of rock climbing not cycling, but the training philosophies are very similiar. i will say when you get it right and figure out the best way for you to train and put together a peak, it's amazing how strong you can feel.