You can expect pleasant day temperatures between 68 to 93 degrees Fahrenheit year-round.  I personally have experienced 68 degrees very few times during all of my visits to Bali.  If one travels to visit the villages in the higher regions (which I highly recommend), it does feel a little chilly...like summer in Michigan!  

Please don't let the 93 degrees be a deterrent...there is air conditioning or an ocean breeze readily available almost everywhere. 

From December to March, the West monsoon can bring heavy showers and high humidity, but usually days are sunny and the rains start during the night and pass quickly. From June to September the humidity is low, and it can be quite cool in the evenings, "cool" being a relative term. During this time of the year, you'll have hardly any rain in the coastal areas.

I have traveled to Bali during nearly every month of the year and have yet to find a "bad" time to be there. :-)