I say, all the way.
I know the answer to this has to be on the forum but after multiple searches, I couldn't dig it up:
I recently had a new faucet installed. It's working fine except that the water pressure is ever so slightly lower. I checked the aerators and rinsed them out. Looking under the sink, the new shutoff valves are straight up and down. I haven't checked them yet to see how much more I'd need to turn them to open them up completely because I'm unsure if it's all right to open them up completely, which would probably solve the lower water pressure. I remember reading somewhere that you open them completely but then back off a quarter or a half turn so they don't stick in the open position as scale builds up.
How far should the water shutoff valves for a sink, tub, or toilet be opened?
I say, all the way.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014
Shutoffs should generally be opened all the way. Many of the are "back seated" to prevent leakage out of the stem.
In most cases, you are correct that opening all the way then 1/4 turn back is what you want. The flow is controlled these days by flow-restricting aerators. Thank your legistators. Many new stops are 1/4 turn and are either open or closed and it's not recommended to use them to throttle water.
Using the stops to throttle water was a reasonable thing to do back in the old days when faucets didn't have restricting aerators and water pressure could vary from region to region. It is still feasible if your water pressure is around 80 PSI, but most municipal systems on flat ground are around 50 - 60 PSI. (It's more of a problem where there are hills and pressure can vary from 15 PSI to 100 PSI.)
Thank you for your replies. I finally got brave and turned off one of the new shutoff valves and then turned it back on and discovered that the straight up and down position is open all the way!! I left it that way.
Did you have shut-off valves before?
If not, and you had copper pipe plumbed all the way to your faucet then you would have experienced higher flow because it is wider than the supply lines you have now.
Just a guess.
Yes, I had shut off valves before and the hot water valve was "frozen" in the open position. No matter how hard I shut it off, water still came out full force. The faucet needed replacing (1 gallon per hour leak!) so I had the plumber replace the valves and the water supply lines. It formerly was copper pipe all the way to the faucet and is now braided stainless steel from the valve to the faucet. So, it sounds like that's the answer for the cause of the slightly diminished water flow!! Thank you!!