TBH, I'd say <10 percent of my clients shower first, and I've never really had an issue. I don't think asking every client to have a bath is practical for either you or them. It would cost you a fortune in hot water for a start, and you'd constantly be cleaning it.
I'd suggest offering them a quick gypsy wash if needed - fill the handbasin with hot water with a squirt of neutral shower gel in it, have a pile of facecloths on the side, and they can wash their armpits, groin and arse if need be.