I’m actually surprised lots of people use tie downs on the stern. I’ve been trailering boats for 35 years and only use stern tie downs for long hauls. I always use a bow ratchet strap to keep the nose locked in which I think settles the boat into the bunks. Youre talking about 5 to 7000 lbs of boat, fuel etc. With this much weight the rear of your boat isn’t jumping off your trailer unless you are hitting deep ruts at 60 mph, which on the roads I frequent does not exist. I’m not saying it’s a bad idea I just don’t think it’s necessary for your standard home to ramp trip on good roads IMO.
Big factor here, is bunk vs. roller trailers, and normal operation vs. accident preparedness. With a bunk trailer, you’re somewhat right; for short trips at moderate speeds the boat isn’t going to move around much. However, if you’re in any sort of accident, those extra stern straps may be the one thing that keeps the boat firmly on the trailer and upright, as opposed to sliding/bouncing off and potentially causing additional damages and injuries. If you’re going above 25mph or so, that’s a lot of momentum; you need straps.
With roller trailers, stern straps, and BIG ones at that, are 100% required every time that trailer moves with the boat on it. If the bow eye fails, which happened to a member on here, those two stern straps are the only thing keeping the boat from rolling right off the trailer. Even if the load is such that the bow eye fails, that same load is divided by two at the stern; they should hold. And if the straps are adjusted correctly, the load should be evenly distributed amongst the three points in the first place.
The type of strap also matters quite a lot: get rid of any of those cam-lock tensioners you’ve got. For road use, the only acceptable straps are large ratchet straps (2”+) with a beefy center ratcheting mechanism. Make sure when you tighten the ratchet, there’s at least one full wrap of the strap around the center ratchet axle to prevent it from slipping under load. Likewise, make sure there’s not TOO much strap wrapped around the center ratchet axle, which can cause the latching bar to get pushed away from the ratchet pawls, so that the bar doesn’t fully latch down.
Also, read the fine print on the packaging. In some cases, the SWL of the strap may be significantly higher than that of the ratcheting mechanism. They have to be listed for liability purposes, so be sure to check just in case.
