Many people who have visited Bali think that September is the best month to go for holiday here in regard to weather. It's true that there time for hotter and cooler temperature, lower and higher humidity. However, it seems that September is a period that the temperature is not so hot and not so humid either.
September is the last month of the dry season and in October the rainy season will start. On the rainy the temperature is cooler and it somehow even in September, a month before rainy season, the temperature has become cooler. The air is just feel nice (again, not so unbearably hot). There will be occasionally some rain pouring on a day or two just for a couple hours even in September. I think this is just a prelude to the rainy season. But when the rain has stopped, the day becomes cooler.
If you look at the weather forecast for September, you will see that it will be mostly cloudy, partly cloudy or partly sunny. But then again it will not rain all over Bali, it maybe or local rain in certain area. For example, it rain in Nusa Dua but it does not in Ubud vice versa. It means that if you are in the right place, you will still be able to see a lot of sun shine in Bali during September with low humidity and cool temperature. Now, how good is that?