When I make quiche (and for a while at an old restaurant I was baking them almost daily) to which I am going to be adding anything other than the actual custard and flavouring, I look for three things: will these other ingredients release a lot of water when cooked? Will these other ingredients release a lot of fat when cooked? Will the other ingredients be fully cooked (if desired) in the final product?
In the first instance, most vegetables will release a significant amount of water when cooking. This has two unpleasant side effects: first, it creates steam pockets around the vegetable, preventing the custard from adhering and leaving empty spaces. Second, the steam itself can mix into and loosen the custard, preventing it from setting properly. Neither of these, as Brown would say, are good eats. Of course some vegetables let off more water, e.g. tomatoes, than others.
Second, dealing with fats, we generally have meats. Many meats will release fats while cooking, a process known as rendering. Bacon is of course the fattiest of meats you are likely to use; sausage would be similar. The release of fats into the custard will have many of the same effects as the release of water. In addition, quiche is often served cold, and excess animal fats feel unpleasantly greasy in the mouth when cold; unpleasantly oily and greasy when hot. Cheeses are not, for reasons I do not fully understand, generally a problem when the release their fats into a quiche. That being said, I have never tried baking a quiche with very fatty liquid cheeses such as brie or camembert; cream cheese and the similarly textured soft goat cheeses present no problem at all.
Finally, we must consider whether the ingredient(s) will end up cooked in the final product. Custards set at quite low temperatures, so this is an important safety consideration when dealing with meats. It is less important for safety with vegetables, and becomes an issue mainly of texture; do you want your vegetables to be crisp and crunchy or soft and limpid?
For all of these reasons, I always cook meat before including it in a quiche, unless it is charcuterie which is relatively low in fats and is safe to eat without further preparation, such as prosciutto (but not, I'd point out, jamon Iberico, if cut traditionally by hand, as it so often includes so much glorious fat that just melts lusciously... I digress). And I almost always cook vegetables; tomatoes, spinach (and other leafy greens) are always fully cooked to remove as much water as possible. Other vegetables depend on what I feel like. I generally will only blanch broccoli long enough to set a vivid green colour, as I like the contrast of soft custard with the crunchy broccoli. Your tastes may vary, of course.
Meat: always cook! render out excess fat!
Vegetables: always blanch at least! cook out excess water!