: It seems that it would be good for every fillet to be as strong as possible.
: Obviously some will be under more stress than others. But, how can I
: predict which ones?

Unfortunately, there are no easy answers. I don't think the chine fillets of a S&G need milled fiberglass reinforcement if you are going to glass over them. If you are not, maybe it would be a good idea.

I will sometimes add chopped fiberglass when I do the fillet around the coaming, partly because it makes a reasonably translucent fillet and if you sit on the back coaming when getting in you concentrate a lot of force on the coaming.

The best answer I can provide is to think about the loads a joint might get and use your judgment. In most cases regular wood-flour and cab-o-sil will be strong enough, add chopped glass when you want a little extra security.