My brother in law is a car salesman and says the time of the month doesn't really matter. The salesman are under a ton of pressure every single day, they'll make the deal whenever they can get it.
I imagine every dealership is different. There are many kinds of sales managers, some will let things slide at the beginning of the month and crank it up at the end, others will ride you all day every day. It just depends on what its like where you go.