Most posters around here are pretty old hat, but for recent arrivals or non permanent residents, before you leave your home country get travel debit and credit cards with no fee. I know they are available in the United States and Australia. In almost every Asian country I have traveled in, the benefits far outweigh the local atm fee. For Indonesia, I tend to get a 200 idr rate better than the published rate in google. It takes the hassle out of changing money and dealing with banks or money changers. No arguing over creases in bills and no carrying around large amounts of cash. Also, it takes away having to change money at the airports. I have seen the airport rate 1000 idr off.
The law in Indonesia is all transactions are to be conducted in IDR. When possible, stick to that or remind people of that. It seems like when prices are quoted in a different currency, such as $, there is an increase for harga bule. This can apply to all countries to stick to transactions in local currencies. Also, I have met more than a few Indonesians that still shorthand estimate 1 usd = 10,000 idr. It has not been that for few years.
Staying on this topic then.....when you pay with a credit card and you are offered home currency or local currency, always chose the local currency. The exchange rates used by the credit card machines are terrible and usually a few percent worse than your bank exchange rates.
Very often goods imported directly from the US, get quoted/sold in US$. They don’t want a currency exchange risk. Most popular items are laptops and high end computer peripherals. Mainly in tech centers as Mangga Dua and Singapore.
Something completely normal for US citizens but which can be a nasty surprise for visitors from other countries, are the taxes which are added to your restaurant bill. As well as a service charge. This can easily be 15%.
So we get to tipping. Esp. with a service charge, not so many people leave a tip. And if they do leave something on the table for a waiter or so, it can be astonishing low, like 5.000 rupiah.