I just read an excellent Japanese-instructional post about もったいない and 申し訳ない. The author then relates what her parents used to say when she would be wasteful (もったいない) and not finish her rice. They would say
That is, they would apologize very politely to the farmer (who had worked so hard to harvest those grains).
This reminded me that, while in Japan, a fellow expat gaijin told me the adage that is the title of this blogpost. Thus, I try never to spill rice when I'm pouring it, and I always try to finish every grain.
(... even though I have no idea if this is an actual saying or not.)