That is, of course, the 64K question of bunkering, is it not?
For the most part, I NEVER build a bunker on a downslope. I have seen it done, but by the time I build the 4-5 foot of support fill from the front, then it may be a 15-20 foot fill on the back, if the downslope is steep enough. It always looks fake.
However, on basically level ground, I have no real problem adding fill to build a bunker, if really necessary. After all, we are trying to use the natural features where we can, and artificial bunkers are usually added where there ARE NO NATURAL FEATURES to be had. So, I expect that I will have to build some on flat land.
I am not even sure I care if the fill is obviously constructed, although I don't like it to be so obvioulsy artificial as to jar the eyeballs. In one way, just accepting that you have decided to build a bunker for hazard is just as "sincere" and "minimalist" as deciding you have to keep adding fill all the way down the fw (or substantially so) just to try to tie it in and make it "more" natural.
Thus, that leaves the question of just how often do you use the natural up slopes (perfect for making bunkers visible) for bunkers, vs no hazard, accentuating the landform into mounds, or some other thing. I tend to look at every up slope as a great sand bunker location, and start the process of talking myself in or out of it.
I do look at how close it is to a real landing zone so as to affect play. That said, if the DL is 300 from the tips, and I expect the tee shot to land about there for the "average" back tee player (not affected by wind, slope, etc) but the knob is anywhere from maybe 270-320, I would probably put a flanking bunker in the most natural location, if I decided to put one in. There is no one who hits it exactly 300 yards, and even a single player whose average tee shot might be 300 yards would probably hit a range of tee shots from 280-320. So, no sense obsessing over a bunker at any particular yardage.
On the other hand, if the slope is under about 270 from the tips, I look to see if it can function as a carry slope (unaided) or carry bunker, if the visuals are right. Again, if its shorter, I don't care - I just open up the potential carry to more players on a daily basis, even if the margin for error for some gets a bit bigger than might be ideal. Of course, we can play with tee locations, too.
Its a circular process and the decision probably can't be as clearly right or wrong as anyone might type out here.