Similar thoughts to what HadesNL shared. Now I do have a bank ATM card that doesn't charge a withdrawal fee, but I also feel more secure when withdrawing at official bank ATMs in airport or the bank branch or a major mall.

Now not sure for what you'll need the cash, but we pay most things with credit card which are widely accepted outside of small shops and whole in the wall type eating places. So usually we only take 1.5 - 2m IDR and that is plenty for 1 week stay. Car hire is via Grab or Bluebird (taxi) app and also charged to credit card. Bigger spas (outside of hotel) take credit cards, supermarket as well as do many restaurants and bigger shops.

With the small money changers on the street, which I wouldn't use, rates are better but make sure they acknowledge if you give them say 100 USD (some claim lesser amount given later), don't go alone and count your changed money calmly and thoroughly as those guys are very good at swapping bills, folding them in a special way to double count, etc. Now don't take this that all money changers in Bali are crooks, but there are enough reports of unsuspecting tourists who got short changed.
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