You NEVER want to tile directly to dimensional lumber, and it is to be avoided tiling to plywood. You really also want some waterproofing there. You should leave enough room to install either a tile-able membrane, or 1/4" cbu, and if you use cbu, I'd use a paint-on waterproofer like RedGard from Custom Building Products on top of it. The ply should be made of C grade or better faces (i.e., no voids) and exposure 1 glue (water resistant). There will be some inevitable splashing over the lip, and you don't want things delaminating or rotting as a result.
It will look much better if you tile, then install the tub so the edges are underneath the lip. For good help on tiling, check out www.johnbridge.com.