Going back to 1974 when the overtime rules were established (this includes older rules where if you kicked a FG, you win the game), 52.0 % of the teams that win the coin toss actually win the game. 43.4 % of the teams that lose the coin toss win the game. 4.6 % ended in a tie.

I do not have a breakdown since the rule changed that both teams get possessions unless the first team scores a touchdown, but that likely evens the odds even more