I always do, unless I'm in a total rush and I just do the top lashes with a quick look. To me at least, I feel like it completes my eyes. Not everyone feels that way I'm sure, I know lots of other ladies only do it when they want a more dramatic looking eye.
I often put blue mascara on my botton lashes. I love it!
Oh, and since I do have long eyelashes I put black mascara on my bottom lashes so it's not all focused on the top eyelashes.
I always have mascara on my bottom lashes. It just makes my eye look better and more complete, like someone else said. I do dark smokey eyes a lot so if the bottom lashes didnt have mascara on them they would look so bare. It just makes your eyes look more finished.