I don’t think it would work well as a preventative or have any significant effect unless pellets give your rabbits a lot of gas (in which case, considering changing pellet brands is advisable). Simethicone is effective because it makes gas bubbles combine in the stomach so they past through better (says the internet), so maybe using it as a preventative when you add something new to the diet may be a more effective use if you’re thinking about preventative?

Simethicone doesn’t do harm, but to be safe, it may be best just to use it when necessary.