Just to back up what 1200RT said, the only time it really pays to wait is on vehicles that you know are selling like crap and will never recover. My wife wants a minivan. She likes her Avalanche, but we need more seating capacity. Of all the vans we drove, she likes the VW Routan the best. The dealer is currently willing to sell a van that MSRP's at $34,400 for a tick over $25,000 not counting tax and tags (That's about $9,000 off MSRP).
They are essentially selling at invoice and there is $5,000 in dealer cash they are throwing in as well as 0% for 72 months. However, I know for a fact that VW has not confirmed a 2011 Routan yet and the inventory is piling up despite the discounts, there are in fact NEW 2009's on the lots that were converted to certified used inventory.
We have decided to wait it out a bit longer as if VW follows the same pattern they did on the 2009's they will start putting customer cash on the hood ($1k - $2k) as well as boosting dealer cash to $7,000 and retaining the 0% for 72 months. That will work out to a total of $12k - $13k off MSRP and price the van at about $22k.
So, in general it really doesn't matter if you wait or not for the "best" deal. However, there are vehicles that sell horribly and waiting it out, even if you may have less choice can save you some serious cash.