I like to build my own stub-outs.
Stub-outs are the end portions of your water supply that stick out of the wall or the floor. It is these pipes that are ultimately connected tot he faucet, toilet, etc.
While I am a DIY plumber, not a professional, I have a few reasons why I like to make my own stub outs to share:
1) One size doesn't fit all
For the most part I find the stub-outs are not long enough for my liking. I know my limitations for soldering so I just wanted a bit more copper sticking in to that vanity or base cabinet. That way I have a few more options when I come around tot he final connections. Hey I can always cut off a bit of copper - adding more length can be done bu tis a pain in the butt!
2) Make PEX connections in accessible locations.
I was just more comfortable making the transition connection between PEX pipe and copper in a location that I could easily get to later. Having it in a wall behind a vanity cabinet would mean removing the cabinet and cutting in the wall. By having the connection to the PEX in the area between the floor joists meant that I would only have to cut a hole in the ceiling below.
3) Stronger material at the rub
Being a first time user of PEX I was just a little bit concerned about places where the PEX may be subject to wear and tear - namely at the exits to various applications. So I decided to use tried and true copper in those locations
In many of the locations I could have brought the PEX pipe right to the appliance, however, I choose to error on the side of caution.