Sometimes the incentives are either low rate financing OR cash rebate. If you plan on paying cash, then you will do better to choose the cash rebate. Occasionally a manufacturer will offer BOTH rebate and low rate financing - such as when I bought my subaru & received both a 2500 rebate and 2.9% financing. The financing was so low that I used my cash to buy a sci-tech mutual fund at $11 a share the day I bought the car (I had a strong opinion that the sector was undervalued at the time). By the end of the year, it was $22 - so investing the cash, rather than using it for the car, made sense for me.
The negotiation process is the same otherwise. Its easier on a new car since every dealer pays the same. You will not arrive at a lower price using cash on a new car. Check the TMV here by doing a custom appraisal and also check the "prices paid" in the models forum. Nearly every model has a forum in which other people report their prices paid. If you have a trade-in, you also negotiate the price of your trade - you don't just accept the dealer's first offer.