the only time i said no was when i was in line to see a in con concert. it wasn't so much about me missing my spot in the line, but i thought it would be sort of rude to others to hold up the line on this person's behalf.
on the flip side. ive seen people who are actually patient enough to wait for you to finish what you're doing before requesting a photo. one time me and my friend were in line (again lol) trying to get some promotional free stuff. and after we finish this person asked for a moment to snap a pic. he said it was worth the wait cuz he rarely see char from the series i was dressed as. i was kinda confused at first but then he mentioned hes been waiting for us to finish and didnt want to be rude.