Check out this recent thread:
Doing a remodel. Existing 1/2" supply lines to shower. I can replumb with 3/4" supply, but shower head is to a standard 1/2" stubout. Is there any advantage to converting to 3/4" supply line from main, or is the 1/2" a limiting factor? I'm installing ZOE industries Polaris II double shower head.
IF your shower valve will only flow as much, or less, than a 1/2" pipe can supply, which is very likely, then there is no advantage to using 3/4" supply lines.
A 3/4" valve can flow as much as twice as much water as a 1/2" version of similar design. But, whether you need it or not depends on how much water the heads need to provide decent spray. And, unless you have 3/4" supply lines, a 3/4" valve will only give you marginally better flow if fed with a 1/2" line, if at all.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013
A 3/4' ceramic seat valve has openings LESS than half the size of a 3/4" pipe. First you take away the space needed for the rim of the seal, then divide the remainder into four pie shaped pieces. Two of them are open for water to flow and two are closed for shutoff purposes. Then take away the space needed for the webs between the quadrants and you are left with an area about the same, and maybe less, than a 1/2" copper line. Take a stem out sometime and measure the area of the two openings.
I would switch the lines to 3/4" for sure unless you are piping in 1/2" copper.
Compare the inside measurement of a pex fitting 1/2" vs 1/2" copper and you will see a big difference.
If you are re doing your bathroom do it right and re pipe it with a new home run. Insulate the pipes and strap them down.