On a fairly flipped coin, the head is heavier than the tail and more likely to wind up on bottom. Hence, tails is more likely to come up on top. Even on the quarter the difference is far less than 0.1%. The lighter the coin, the less the difference.
When calling on a toss you think is fair, call tails. When flipping or otherwise going for maximal fairness, flip a dime (or if you're not in the US, the lightest available coin).