The invoice price has changed over the last 15 years, due to the internet availability. All companies have inflated the invoice price, and give dealer volume incentives, and other incentives on the back end.
Therefore, you never know which dealer needs to reach a certain volume amount in a certain month. Theoretically if at the end of the month a dealer needs to sell one more vehicle to reach their magic number, they would lose money on that one sale to get the much larger incentive on all the vehicles sold in a certain time frame.
This places the buyer and a greater disadvantage than when we knew the true invoice price. One way to check this out would be to price vehicles on edmund and see when the average sales price is much less than invoice price. If the invoice price were true it couldn't be this way.
In order to get the best price you need to ask 7-10 dealers to bid on your business. It is a lot of work, and you do need to have done your homework, but the main thing to know is it is not a deal until you are happy with it, and the dealer will not make a deal they are not happy with, even if they tell you they are not making any money on the deal (and they will). When they did that with me recently I let them know I knew that wasn't true, and then set to calculating their numbers (which are meant to confuse you, and I have a BS in Accounting!).