"Say Cheese!" apparently means the same thing as "Please smile." It has become a ritual of photography. Gather a group of people and pose them. Place another person behind a camera to take photos of the posed group. Nine times out of ten* the cameraman will start by asking everyone to "Say Cheese!" and nine times out of ten* the group dutifully responds by doing just that (*I made this statistic up). I always assumed there was something about the word that automatically shaped your mouth into a smile. Recent experimentation, however, has suggested otherwise. I have found myself capable of saying "cheese" numerous times without ever smiling or appearing to smile. I can even hold a pretty decent frown while speaking this word. (On an unrelated note: my coworkers recently sent a petition to Office Services to improve the sound proofing on my cubicle or move me into an office of my own. They're such great people).
So where did this idea come from, that saying the word "cheese" is the best way to get people to smile for pictures? I don't really think it's the idea of cheese itself that is supposed to do the trick. After all, it does not carry a whole lot of inherent humor value, like orangutans or kumquats. "Cheese" isn't exactly synonymous with "fun." As for inspiring warmly pleasant thoughts, it is not generally the first thing one would think of for that purpose. I'd go as far as to say that cheese is a rather ambiguous food. Most people are okay with it, some are not, but neither its supporters nor its detractors are very vocal about their opinions, for the most part. Thinking about cheese does not generally inspire the delight of, say, thinking about chocolate or maybe thinking about sex. But then again I do know some people who are militantly opposed to chocolate, and some who are militantly opposed to sex. I do not think I know anyone militantly opposed to cheese. Maybe that's why it's so popular as the "please smile" word of choice; it's not likely to offend anyone.
Although, it does still carry a certain amount of risk to ask people to think about cheese. The category of cheese is quite broad and contains more than a few examples which, instead of inducing smiles, might induce wrinkled noses, depending on what type of cheese one chooses to think about.
It just seems to me that there are better alternatives out there. The photographers (the good ones at least) who take family portraits seem to have caught on to this. Many of them have various toys and silly outfits to inspire smiles and smirks in their subjects. They have also come up with more unique and funnier things to say than "cheese." For instance: "pickle nose." When I was little, my brother and I had our picture taken with our Grandmother. The photographer's phrase of choice was "pickle nose," an amusing concept. I am smiling in the picture. My grandmother is firmly holding my brother's arms to his sides and he looks slightly confused. This is because he thought the photographer was telling him, "Pick your nose." That picture took several tries. Perhaps it really is safest just to say "cheese."