Front bearings are sealed, I leave them that way. After 10 races last year I only had 1 that was just starting to get crunchy. The rear bearings I use have a metal shield, rather than rubber seal. I hit them with brake cleaner, blow dry, and re-lube after every weekend. Had no issues with rear bearings last year. All get changed at the start of the season regardless, but I also keep spares in the trailer.