A couple of thoughts - there is a difference between doing the "right thing" and doing "things right".
If the airspeed is going below minimum and the PF is making no correction then it would be appropriate to call "Airspeed". If the same applies but the PF is making a (sensible) correction then no need to call. However, this assumes some basic experience and maturity from the PNF! However, this is also something which needs to be taught during training.
One way I used to tackle this was to reply "Roger" when I was aware of why the call was being made and "Thankyou" when the PNF had drawn attention to something I had missed.