If you choice heads, you are following all the others choice and show that you are not individual minded.
If you choice tails, you show you don't follow the rules or others choice and somehow that you act like a rebel.
It's pretty hard to describe, but this scene is actually important.

Ok, i played this part… and i don't get it… what's with these two asking you to trow the coin and ask if it was head or tails. Also what relations does that have with twilight and Dusk being surprised?