Tipping in Bali is appreciated but it's not required. However many local workers in Bali don't earn very much, so even a small tip really makes them happy. You should never feel that you have to leave a tip, especially if you received bad service and you should not feel pressured.
If you don't know how much to tip, here a small guide
Hotel Bellboys 30-60.000 IDR. They expect usually a small tip.
Restaurants 5-10%. Not mandatory like in US but appreciated. A good way is you can leave the change which you usually get back after you pay the bill on the table.
Spas 15-50.000 IDR. They are usually local private run and are happy for a tip
Hairdresser 5-10%. They would expect a tip for their services
Tour Guide 50-100.000 IDR. He will not expect but happy to accept
Taxi Driver They make tips by themselves. If the fair is 28.300 and you give 30.000 IDR they wont return you the change with the excuse that they don't carry change.
Villa Staff 40-100.000 IDR. Depends on the length of stay. They don't expect but are happy to receive a tip. Villas often have a jar you can stick the money in, this makes sure that the tips is equally shared between the staff
The smallest tip you can give is about 5.000 IDR from there your can go up.
If you tip in Bali they appreciate it discretely, for example to role the money and give it into their hands while a handshake.
You can also brake down your tip trough out your experience, this will improve the service all along. Don't tip before the service begins this would be seen as bribery and its not welcome in Bali.
Hope this makes your more confident on your holiday in Bali.