The sides would be the next in holding power. I have taken out many a neck in which there was almost no contact between the bottom of the neck and the mortise yet it was still holding under tension. The leverage factor and also that the bottom of the neck is end grain adds to this being the weakest link.
Don't be such an elitist re screws. I've done it plenty of times-don't worry, not on your basses!
...and of course not on any carved bass but sometimes its the only cost effective way to get a Kay or something similar up and running. Generally you see only amateurs using metalwork but a skilled luthier can screw with quality.