I put bilge pumps in all of my skis. I don't think it's necessary but one day could be the difference between a broken ski and a sunken ski.
I have heard stories of people out in the ocean who undoubtedly would have lost their ski if the bilge wasn't keeping it from going under.
I think it's a cheap peace of mind that can only potentially help.
I used a manual on my first ski and an auto on my second. The auto pump is wonderful and I would only install them from now on. You just flip it on and it checks for water every 2 minutes.
Rule pumps are by far the best and consequently more expensive but worth the price. It's an easy install just be aware you will be punching a hole in your hull.
I like to place my bilge directly in front of one of the bailers as it's the lowest spot with easy access. Just put a little marine epoxy on the hull and epoxy the mount down to the hull.