I was involved in the testing that produced the information Jim wrote about.
Basically if you touch the string to the side of your chin you can change it's impact point at 18m by 4 rings on a 40cm face.
To test this I shot 3 shots on a 3 spot.
Normal: X
Aligned the dot onto the left side of the peep: Left 10
Touched the string to the side of my chin: Left 7
This is quite dramatic and while some people will shoot fine with face contact, it will lead to odd shots they can not explain.
Pat Coghlan use to be so hard into his face the string was bending and while he was a 1390 shooter it wasn't till he got the string off his face did he break 1400.
You can still touch your nose, but keep it on teh front or the left side, not against the right side, and be careful not to bury the string into your face.
For example this is bad
As is this
This is not too bad, she would just have to be careful
Cousins here while quite solid is opening a clear path to the string. Notice how it's more on the front of the face than the side.
This one is excellent. See how the string is mostly off the face and any contact is very very minor. I believe he has a 1400 as well.