My professional opinion as a classically-trained chef is absolutely not. If you boil bell peppers the water will leach out nutrients like Vitamin C. You can also make the bell peppers a bit limp and water-logged. A soggy pepper will not caramelize, or brown, in the oven. And this caramelization creates new, delicious flavors. But it will only happen in the absence of moisture. Soft peppers will also not be easy to stuff. If you prefer to cook bell peppers before stuffing, the best method is to roast them in the oven. But only do so for 10 or 15 minutes around 375° F. Since you will be putting the peppers back in the oven after stuffing, you risk mushy peppers if you cook them too long before filling.