Nipple pinching, as distinct from checking for discharge, is more for those who like to say 'Ooh, it hurts so good!'
No harm in that of course, provided nails are clipped, Also, if nipples are clamped as Lady Teddy describes it should be for a maximum of 10 minutes. It can be repeated when the blood has been allowed to flow again for a few minutes.
For the purpose of checking for discharge, simply squeezing or pinching is not appropriate. Most people forget that the nipple has 'roots', the milk ducts, and they radiate out from behind the nipple into the milk secreting tissue.
The proper way to check for discharge is to grasp the breast tissue from behind the nipple in an area approximating to a large areola, preferably with oiled fingertips arranged in a circle and draw the tissue forwards, slipping finally over and along the nipple. Do this at least three times.
The purpose of checking for discharge - well it has been shown, in severe cases, to be an indicator of possible brain tumour! Yes, indeedy!
It's also a good method, beside stimulating the breast tissue as a whole, to be efficacious in trying to make a non pregnant woman lactate which is something some people are interested in.
In my examinations I'm always at pains to tutor women who don't check their breasts regularly and a lot that don't check their vulvas ever!
If you find a problem, then whilst it's not exactly a good thing, it's sure not a bad thing either.
If you don't find a problem then its a relief and whats more its mighty pleasant too!!!!!