cement board sits on tub ledge
There are two reasons to not have the wallboard touch the tub lip:
1. To prevent stress and cracking due to expansion during temperature changes,
2. To reduce wicking of water up into the wallboard. This is probably not a problem with cement board as opposed to drywall but a gap of less than 3/16 inch will not prevent wicking. Wicking problems are further reduced by proper maintenance of the caulking around the tub edge. Concrete is porous. The danger of continuous wicking is mold impregnating the cement.
The disadvantages of crab apple trees. In summer, the apples are too sour to pick and eat. In winter the birds come and leave dropping all over the place.