I went to Bali earlier this year.
The smallest denomination I had was a 200 rupiah coin.
Laurence is right, 100 rupiah is virtually worthless.... expect
to have your change rounded down to the nearest 200 rupiah where
applicable.
E.g. if your exact change is supposed to be 1100 rupiah, then
you should expect to get 1000 rupiah instead. That was what I
experienced.