In my experience it don't leak there , it leaks where the valve meets the copper pipe , then you've got agg, no matter how much u tighten up, I've known the valve to be only just on the pipe and the whole thing come off
when i am quoting for the work i have a look at the valve and the nuts, if they are covered in paint i give to work to our local jobbing plumber, if they look nice and new with no paint then i do them myself.