I think the answer depends on your body so there is no "right" answer.

Personally?
I ALWAYS carry food, and I ALWAYS carry one more bar than I think I'm going to need. This is mainly due to the diabetic history of my family and my tendency to run out of sugar and be in deep trouble when I'm exercising (hyperglycemia more or less).

I used to only carry water, but since it's been hot over the summer I've been craving salt after rides (hint: if you are craving Gatorade, chips, etc. your body is telling you you need salt, you should listen). So I got some HEED to try and now carry one bottle of HEED and one of plain water. I'm noticing a huge difference, so I'll probably keep that up even when it cools down a bit.

My typical rides are 45-75 miles.