I forgive TKs if I was doing something stupid, like firing back toward my team (even if I was shooting at actual targets), accidentally running in front of a friendly I didn't see, or stepping into a room to see a 'friendly' nade go off.
I usually I don't forgive TKs for things like, getting shot in the back by a teammate while I was in a cap zone,... with my back to them, them throwing a grenade to 'clear' a room they didn't even look in when I've been holding it for several minutes, etc.
The idea of "Always forgive!" is beyond stupid, and if you do something stupid, then you deserve to take the results of your actions.
Ideally I would like to see the "type np to forgive" to become "Press F4 to Forgive, F5 to report." If you press F4 after you die, then the other person doesn't lose points. You did something stupid, it was your fault, etc. If you press F5, then the person loses points.