A photographic print is usually printed at 240 dpi or higher. As you point out, for a very large print a lower density may be fine because the viewing distance is expected to be further. Keep in mind, however, that you can't always frame the shot in the camera the way you want it to be cropped for the picture. So you might need to make every pixel count. Also recognize that your monitor is about 75 dpi which is much lower resolution than what you would want on a print. I usually try to print with at least 150dpi in the original print, even when printing at 16x24. Note that the printer software will resample so that more dots are being printed than what you have in the picture - but your resolution is still limited by the number pixels you have in the cropped file to print.