Generally, I find that teams feel much more comfortable with estimating bugs and the idea of only measuring "new stuff" doesn't provide such a great psychological effect as it's supposed to. I used to have a strong opinion about this in the past and insisted on not estimating bugs. Over time I've come to realize that what's more important is keeping your estimation as transparent, clear, and consistent as possible. So, generally, I estimate bugs with my teams. In the end, forecasting is a manager/Agile Coach's jobs and if it complicates things for you but makes it easier for the team — that should be your preferred course of action.