I purchased some macarons for a meeting at work the other day and they were so colorful that I was compelled to take out my iPhone and snap a photo. I didn’t realize that the flash was set to automatic when I took the photo and the result was absolutely ghastly. The two photos show the same macarons photographed without flash (top) and with (bottom).
This is the same result I’ve seen with onboard flash units on DSLR cameras. The color temperature and angle isn’t complementary to the subject being photographed, and it can completely ruin the look of your shot.
So, my answer to flash or not to flash is: Don’t do it. Either improve your lighting conditions or bump up the ISO on your camera so that you can capture the image you want without setting off a flash. I know that there are always exceptions, like using fill flash or a studio shoot with a professional lighting rig. But as for using the onboard flash on your phone or on your camera, don’t do it!