The great thing about the Bali weather is that it is fairly consistent. I do not think there is really a bad time to go, unlike other islands or countries in the region.
The temperature usually sits at about 28 degrees on average throughout the year, although at certain times, during European winter, the humidity can be sometimes quite high. The rainy season is from December through to March, but the rain tends to be a sharp downpour late afternoon or at night time. Before too long, it is brilliant blue skies again and the air, a lot fresher. The driest time which is also the high season, is from about June through to September.
The further inland you travel, the more lush and mountainous it becomes and therefore has a greater risk of showers at any time, especially by the mountains. If you are going to travel inland, it does get a little bit cooler in the evenings, but its all relative and coming from northern Europe, it has never bothered us hardy folk!
When there is a sudden downpour, it can be an absolute deluge! Shop keepers and cafe owners count this as a blessing and are usually very accommodating in letting you take cover, of course, with fingers crossed that you may buy something!
Even a deluge of rain is part of the holiday and often entertaining! In an instant, the roads can be flooded. If you are on foot though, when everything is under water, be careful as the pavements, kerbsides and drain covers are not exactly uniform or predictable!