My last boat was on a trailer during the off season. By launch time I had removed water by the bucketful. During the season I kept it in a slip. Barely a drip, even after the heaviest rainfall. I suspect that the boat, designed to be in water, flexes differently when stuck on a trailer, whence more points of ingress that didn't occur when it was stored in the water. Since I was more concerned about what happened when it was in the water where it was supposed to be, I left it alone.
In one of the first years with my boat I did not get her winter cover on for about a month after pulling in the fall. I noticed that a few ports were leaking, and they never leaked in heavy storms or from green water over the bow. Opened all the ports and closed them again. It was the flex of the boat when the rigging came down that caused the leak. Sure glad I did not FIX all those seals.