went to Maui in Sept. for my honeymoon. It was not packed at all during that time ( sept 17th to 27th ). May is a month shy of the big summer boom so you should be ok.
I stayed at the wifes timeshare place in Kihei, very nice quiet area but a bit of a drive to the more nightlife side near kannapali and Lahina. Depending on what your looking for more, not a lot of nightlife or a party area.
I dont know what Machado is talking about but Wailea is like the Beverly Hills of Maui, much more expensive of an area than the Lahina/Kannapali area. And there is no Airport near Wailea
I've been there a couple times, let me know if you have any questions about stuff there.