Yeah; I think I might make the ability to ride sans roll attached to your background (so if you're Fremen, it's routine to you and a perk of being born on Arrakis). With offplanet characters, they gotta do the ritual... But probably only once, then after its routine. I think I may simply track "worm tameness" or some such, and call for a steering roll of some variety if it gets exhausted or enraged.