Although Hilton is not currently offering an online booking bonus, the few times I have first called to check prices without booking, then checked the rates online, I found there to be a savings of at least $10 online versus booking directly with Hilton.
Also lets not forget the double base bonus promotion for stays booked online and paid for with Visa from 6/ ~ 8/31.
http://www.hiltonhhonors.com/visa2x/
That's reason enough for me to book all of my Hilton reservations online.