Sink Angle Valve: Fully Open or closed Tiny Bit
These are 1/4 turn or ball valves, turn them fully on or off for best performance. Think of a bearing with a hole drilled through it. This is what a ball valve is there are 2 seals one on each side when the valve is turned fully off there is no way for the valve stem to leak when turned fully on again there is no pressure on the stem. At 50% open you do risk a leak but there is a seal it is not robust because the only time it is needed is when opening and closing. 

With multi turn or gate valves we fully open then close a little to relieve pressure on the valve stem packing.
This may not be as big a deal with home plumbing but really makes a difference on high pressure valves like SCUBA , oxygen, acetylene or other high pressure gasses. 
Opening fully and turning 1/2 to 1 turn closed will extend the valve stem packing life on a gate valve. 
As far as closing I always fully close the valve when off I don’t want it to leak.