I've been doing pretty well without a lite app, although it could be a major factor for future sales once the initial hype dies down.
If your game is $0.99, $1.99, or $2.99 you can probably get away without a lite version for a while. If it's an app with some utility value instead of a game, my experiences may not apply... but I would think you can charge more for good applications (look at Jaadu VNC and RDP examples of more expensive but well done apps) even without a lite version.
Of course, make sure you have a lite version ready to go sooner or later! I made the mistake of not considering that, and after I'm done being featured by Apple I may need to work on one to stay up in the ranks
My final thoughts- get your app out there and see what the initial sales are like, then release your lite app a week or two later. Then stagger updates to it and the full version so that you keep one or the other visible based on Release Date in iTunes (assuming they ever get that fixed!).
