Old-style nailed Blues Harps or Marine Bands: don't soak or wash. You'll have to un-nail them and clean all the non-wood parts, doing what you can without water to get any crud off the wooden comb, then reassemble. You could sterilise everything with isopropyl alcohol if it makes you feel better. Hohner harps with "MS" stamped on the covers, which have parts screwed together, may have plastic or wood combs, but the wood is not the same as in the old, nailed harps and does not swell to anything like the dame extent. I've found that a quick rinse-through with tepid water, then a prompt shake-out and tapping to dry, then a night out of the case, does them no harm. Plastic-combed harps can be washed in the same manner but you don't have to be quite so fussy. If you do this after every gig, leaving them to dry overnight, you will avoid the need for any more drastic cleaning and your harp will sound better. The exception to this would be harps containing valves (which most 10-hole diatonics don't have, and you'll know if yours has them because you will have consciously chosen them and paid a lot more). You'll have to remove the covers and plates if they get really nasty, but if you wipe clean with a damp, warm cloth after playing you will never have to do this (with luck). Don't play if there is any chance that there is food residue in your mouth. It's going much too far to suggest you shouldn't drink beer either. One does have to live.