I agree to get a size smaller only because I know I wouldn't be wearing jeans still even in September.
For me, jeans don't become regular wear until maybe October when the weather cools. It depends though. I personally am holding off on Fall clothes until the cooler weather hits because my body changes so quickly.
When school starts in the Fall (I work in the schools), I plan on getting a pair of khakis and maybe blue or black pants that fit to hold me over until Fall because I know I'll be much smaller this season next year.