Unless this will also have a large tub, then you would have no benefit from a 3/4" valve with one showerhead. Now, running 3/4" up into the area would help ensure there was enough water for that bath, but it won't make much of any difference to the individual valve for the shower. To maximize things, you could reduce it to 1/2" at the valve, but that probably wouldn't be necessary either. The longer the run, the more friction, the lower the flow and pressure while running (pressure with no flow would be essentially normal). But, if you run 3/4" to the immediate area, the reduction to 1/2" for the valve should be fine.