Do you pinch off dead flowers during the summer to force new buds or just leave alone?
Butterfly Bush Pruning
Certified GKH Gardening Expert
They do not need to be, but they can be if you do not like the way they look. Sometimes, removing spent blossoms can force the plant into a second bloom. Deadheading your butterfly bush will not affect its health. In fact, it's good for it. You can learn more about doing this here:
You might also want to consider pruning the bush to keep it in shape, though it isn't always necessary. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-pruning-how-to-prune-a-butterfly-bush.htm