When using using 3/4 PEX for cold water in a bathroom that has shower, sink and toilet, do I need to reduce to 1/2 when I am branching to the fixture?
&quot;Need&quot; is defined by the connection you want to make. As you've discovered, shutoff valves (supply stop valves for use with a fixture) with 3/4&quot; inlet practically don't exist. Therefore yes you'll need to adapt the plumbing to the inlet of whatever shutoff valves you can find. Shutoff valves are widely available with inlets of either 1/2&quot; female iron pipe thread or 1/2&quot; nominal compression (the latter is called 5/8&quot; tube size by some). With those two sizes as your destination, it's time for another visit to the &quot;which fittings are readily available&quot; department.
A copper stub-out is a common choice. It crimps to the in-wall PEX tubing and stubs a section of copper pipe/tubing through the drywall. After the room finish is completed the capped end of the pipe is cut off and a compression inlet shutoff valve is installed. You'll have a hard time finding a 1/2&quot; stub-out that fits 3/4&quot; PEX, though, so that indicates you'll next have to hunt for something like a 3/4&quot;x1/2&quot; PEX reducing tee, elbow, or coupler.
Another possibility is to arrange to emerge from the wall with PEX tubing directly. In this case you might get a 3/4&quot; PEX x 1/2&quot; male pipe thread adapter and pair it with a 1/2&quot; FIP inlet shutoff valve.
There are few rigid requirements in the practice of plumbing -- mostly it consists of following a few principles and getting the job done with whatever parts are readily available.