Yes, a reasonably proportionate response on the use-of-force scale is part of a good self-protection strategy. I addressed that in my post, stating that it was not OK for a man to break a woman's face because she slapped him.
In my state, however, as in many others, you are not limited to proportionate violence in self-defense, but rather to equal OR necessary force... which means the same level of force, or the level of force above that necessary to STOP the attack (usually by stopping the attacker).
I've dealt with old folks who weren't really in their right mind, including one old lady who tried to hit me in the nads because she didn't want to go to the hospital. She was old and fragile, and naturally I used only that level of restraining force needed to keep her from being successful in her resistance.
Being proportional in your use of force is, however, something of a luxury. That is, if you have plenty of force available to you (as I do) then you can afford to be proportional in your response.
If, on the other hand, YOU are a 90 yr old man or other relatively fragile person, you have to react decisively to the threat of violence. For instance, if a 20yo bruiser balls up his fist and advances threateningly on a 90yr old man, in my state the 90yo would be justified to shoot him dead and probably wouldn't even been charged.
But let's get back to a more realistic scenario. Let's say a 90 lb woman and a 200 lb man are having an argument. The 90 lb woman starts pounding her fist in the 200 lb man's face, repeatedly. You don't expect him to stand there and take it do you? No... The man tries to grab her arms to stop her, and she struggles. He has a strong grip... you know what happens? She gets finger-shaped bruises on her arms. The cops come, see the bruises, and arrest the man.
This happens frequently. It isn't right, and it needs to stop.