For me, colors and numbers are like that. Anything chromatic, be it M&M candies or colored pencils, must be in rainbow order. And while I'm not super strict about it, I love multiples of five and three. I agree with the TV volume thing - I vastly prefer a multiple of five when I'm adjusting my sound; other numbers are ~okay~ but I can't live with it being a prime number (that's not, y'know, five).