There are a lot of hotels in Bali and Lombok, so I think you will be fine without booking in advance. However, I would recommend that you do otherwise, unless you're willing to take the first hotel you see. I once did this in Jogjakarta (no booking) and won't do it again. It's a waste of time, going from one hotel to another to find the best price.
Searching online is more comfortable. You get to see pictures and read reviews from other people. As Willeke pointed out, there are rooms with no or low cancellation cost (until a couple of days before arrival). I went to Lombok and Sumbawa last January and booked all hotels in advance.