Many builders set tub and shower valves early in the construction process before the finish floor is installed, but I think it’s better to have the finish floor run underneath the tub than stop at the edge. This helps prevent water from penetrating through the inevitable seam that will open up and into the sub floor, where it can cause rot. Besides, a caulked joint here looks terrible. This will mean taking extra care of the finish floor and will complicate hanging drywall, but it will result in a better looking and more waterproof bathroom.
Hopefully, the valve wall will have been framed to leave an empty stud bay centered over the tub drain. Blocking will eventually be needed to securely attach both the shower stubout and the faucet body to the framing, and it can be either a piece of 2x cutoff or a scrap of 3/4-in. plywood (which is less likely to split). The blocking will have to be set at the correct height for both the faucet and the shower head. A typical faucet height is 30 in., and a typical shower head height is 72 in., but these can be varied according to personal preference.
Setting the blocking at the correct depth in the wall is a little trickier and depends on the particular type of faucet, as well as the type of finish wall. Some faucets have a limited degree of adjustability (sometimes less than 1 in.), while others are more adaptable to different wall-surface depths. Some manufacturers offer extensions, a feature that you’ll appreciate if your client changes his or her mind about a shower wall-surface material, like from a thin Corian to a thick mortar-bed-and-tile surface. Many faucets come with plastic setting gauges that indicate the correct mounting depth for the valve.
A potential problem with one-piece or two-piece tub units and shower stalls is that the valves and stubouts can get in the way of sliding the unit into the framing. If the valve is on a sidewall instead of a back wall, the protruding stubouts will mean you won’t be able to slide the unit into place very easily, if at all. In that case, you’ll have to wait until the unit is in place to install blocking and to attach the plumbing to the blocking because you’ll need to be able to pull everything back into the stud bay to slide the plastic tub or shower unit in place. If the valve wall is an exterior wall (not a good idea in a cold climate), then you may have to access the wall from the exterior of the building.
A typical tub/shower set-ups, like walk-in showers, need to be roughed-in to accommodate their particular water-supply and drain system needs. For example, a tiled shower floor with a waterproofing membrane will require a two-piece clamping drain set high enough to accommodate a sloped mud floor, the waterproofing membrane, mortar or adhesive, and tile. If there will be tub and shower valves, they too will have to be roughed-in according to the needs of that particular installation, and will probably require the expertise of a plumber to ensure adequate water pressure and temperature.