Some of the top guys may, and some of the top guys try to focus solely on the top NASCAR series and not risk either injury or focus off that series. Others (mainly people lower down), go back to the grass roots series for the fun of it. Leffler made his career driving midgets in USAC, so I'm sure he kept up with that since starting his career back in the late 90s.
He always seemed like a good guy, and I feel terrible for his son. Jason posted about him all the time on Instagram.