From what I've read online the weather In Tahiti is usually pretty great year round even in the wintertime.
I've never been to Tahiti myself yet but I really want to someday and from what I've gathered from information online is that the best time to visit Tahiti would be during the months of May, June, July, August, September and October.
During those times in Tahiti the weather is pretty dry and less humid and the weather is usually pretty comfortable.
During the month of May the prices are usually lower than in other months to stay in and visit Tahiti.