I'm not sure, but I do both, most commonly in cursive. Sometimes I get confused and have to stop and think. I occasionally leave notes on my colleagues' white boards - and few people knew was doing it. I was doing a diagram in another room and started annotating it. The victim of my crude caricature yelled "YOU'RE the one who wrote that message in Jack's office!"

I get practice from tutoring geometry. I usually have about 4 or 5 at a time and we're gathered around a table in the library. I have a sheet of paper and go around asking each kid a question and writing their answers step by step until the problem is solved. It takes them a moment to realize that I'm writing upside down and backwards, but it always gets them excited for a few moments and helps maintain interest. One problem - I can write numbers upside down easily enough, but I'm not so good at actually doing the math - multiplying two numbers long-hand, for example. I can do it, but I'm very error prone (more so than usual). OTOH, I like to make some good mistakes for the kids to catch. Keeps them interested and makes them feel like they're helping me.

It started out as just doodling. I would write the entire alphabet backwards, then backwards and upside down, etc. Just something to do while my mind wanders or I'm trying to focus on something.