Unless it is a long distance run with lots of fittings/changes of direction in between, you won't gain much at all by increasing the size. Remember, there will be two 1/2" lines feeding the valve, and only one 1/2" line leading to the tub spout or shower head. Most complaints of low pressure to a shower are either debris in the valve/head or a low quality/low volume shower head.
Lake County, Florida plumber
Springtime Plumbing Services