I often do that. I think the fog on the lenses can help clean my lenses. I don't think the fogs are dirty though they may come from my breath. I seldom take my eyeglass case with me when I am in the outside. So every time when my glasses were dirty, I would blow a breath onto the lenses and then cleaned them with my cloth. Later, I know the best cloth for lens cleaning is eyeglass cloth, so I will bring my case and cloth with me. But I also blow a breath on my lenses before cleaning them.
Sweetie, there is a kind of solution that can help clean the glasses. My optician told me that this kind of solution can kill the bacteria on the lenses and prevent metal frames from corrosion. You can have a try. I bet you will like it.
The water on lenses can lubricate the lenses so we can remove dirty form our lenses easily. That's why some people blow a breath onto their eyeglass lenses before cleaning them. If you don't like this way, you can buy some solutions to help clean your glasses.