It is pretty reasonable to assume that anywhere within a couple hundred miles could have some issues with breathing from ash. Depending on what happens with wind patterns or any directional blast can also make a difference.

If you are travelling to central Java to visit Borobudur -- it could be risky mostly for health issues. The mountains and hiking around there are great, but if the air is filled with ash...

As indicated Bali is a long way away, so highly unlikely to have any danger from Merapi, but could be impacted due to previously stated posts, and / or govt's need to focus some help for the region and less focus on other areas.

It's got to be pretty amazing there. In 2000 I was in that area and did a few sketches from my hotel of Merapi -- which was smoking even back then.