I made this classic fall dessert with this recipe's filling and this recipe's topping and followed the baking directions of the second. The best advice on adding sugar to the fruit part of the dish is to do so "according to taste" (you can use either white or brown sugar) and also to taste test the fruit that is the base of the dish. I've also added pecans or walnuts to the topping (about 1/4 cup.)
I have served this crisp with vanilla ice cream or fresh whipped cream (slightly sweetened with powdered sugar.) Either way is delicious.