If you're playing against an explosive offensive team, the best defense is probably keeping them from getting the ball. I've seen games where the winning team ran only 15 plays (7 for touchdowns) vs the losing team's 44 plays (in a four-point, 40 minute game).
If you need to keep your defense rested (which, in six-man might include a lot of your offense), slowing down the game is an acceptable strategy.
As for being ahead, I always go back to the words of Al McGuire, the legendary Marquette University basketball coach. At some time during the game, if you're the winning team, your opponent isn't the other team, it's the clock. Your job is to get that clock to 0:00 as quickly as you can.
Or, you can use the New York Giants method. And that's to conserve enough time for your opponent to beat you. I'll take Al's theory.