It depends on what your objective is. If you want to measure how many "road-equivalent" miles you have done to judge the progress of a winter training regime then go ahead and count them. On the other hand, if you want to measure how much distance you have spent enjoying the wonderful nature of outdoor cycling, then don't.
Ultimately, though, there can be only one answer: Do what you want, who cares?
Ultimately, though, there can be only one answer: Do what you want, who cares?