I have been to weddings where the maid of honor mentions negative stuff about the bride or groom. On a few different occasions, "I used to really hate her, but now we're best friends" (Awwww!) or "When they first started dating, I couldn't stand him", or "I never thought this day would come, they broke up 3 times before this..."

So yeah. If you don't have anything nice to say, just lead a crowd cheer of congratulations.