When playing dominoes do you have to have 15 in order to score?

no. score in any increment of 5.

when a player goes out with his last dominoe, the other players dominoes are rounded off in 5's and added to his score.When playing dominoes do you have to have 15 in order to score?
There is a game of dominoes called 5s and 3s. You score for multiples of 5 and 3. If you had double 6 and a 3 at either end of the line of dominoes you would score 8 ie 5 for the multiples of 3 and 3 for the multiples of 5. It is the best score in the game.