Sliding in the rain and sliding on dry asphalt is a different feeling. When you're sliding on wet asphalt, you expect a super easy release without needing much effort and a nice smooth slide. When you're doing it on dry asphalt, you need to have good weight distribution and need more of a kick. Some freeride wheels grip up a bit mid slide. Usually after a while, you get a good feel for it and get used to it. I personally feel that it's better to learn how to slide on dry asphalt because you may begin to have false assumptions of how to slide from learning on wet asphalt. Although it's a bit harder, it's better to learn. Others may have a different opinion on it, but that's how I would go about learning to slide.