Those people will never see you again. If you feel like singing, sing! It's who you are. And do you expect a girl to look up and notice you when you walk by with your can of tuna and peanut butter? You'll just pass by awkwardly in aisle 7 while she goes to purchase anti-fungus cream for her vagina. If you walk by and you're singing, there's a slight chance that you seem like a human being and she may even smile at you.