Depends very much on what kind of tree you are trying to grow as a bonsai.
Here's a list of suggested pH ranges for a bunch of different trees:
You will see that many of them lean a bit on the acidic side, but there is a lot of variability. For example, maples (including Japanese maples) would want neutral to a bit alkaline. Ficus would want neutral to a bit acid. Etc...
In general, people need to remember that bonsai is just an art form for growing and styling a tree. What you are growing is not a "bonsai" it is a specific tree, juniper or ficus or cherry or whatever. The requirements of your bonsai are the requirements of that specific tree, for sun, cold dormancy, warmth, pH or whatever.