Deb is right, waking them at a certain time tends to set their internal clock and set their biorhythm for the day. It also sets up a sort of predictability for your day (unless your lo takes short naps when she's supposed to be taking a long one!). Think of it this way: if you get up at a certain time every morning regardless, eventually your own "internal alarm clock" gets you up automagically that time before the real alarm clock goes off. It only takes a few days. Babies work the same way.
I try to get mine up by 6:30 every morning. However, occasionally she'll wake and want to start the day at 6, and sometimes we don't wake up till 7 and she's still sleeping. So really, as long as most days are relatively routine, it doesn't harm them at all to go out of routine for a few days. But I've found the more routine your normal days are, the more likely you'll have "good sleep days", although nothing you do makes every day perfect.
Just my 2 cents
*HUGS*
Sonya =P