Rommel,
Not trying to argue with anyone either. It's your trailer - So you should be the one making the decision. I'm just voicing my opinion and going from experience. Great thing about advice - you can pick and choose whose advice you wish to follow... or just ignore all the advice and do what you want to do.
I think about it this way (as it was explained to me by the trailer manufacturing company that built my replacement axle): If you put the right axle on the trailer to begin with, you don't have to "preload" it to adjust for future loads. Plus... how can you be sure that axle will unbend later with the fluctuation of weight?
Take for example, an unloaded boat vs. a fully loaded boat that has a 30 gallon built in gas tank (Like the 750 in this example). With gas weighing approximately 5.8 to 6.5 lbs per gallon according to the US Dept of Energy... how much extra weight is that? Lets use the lower weight to be conservative. Multiplying 5.8 lbs x 30 gallons = 174 lbs. Now are you using a 12, 24, or 36 volt trolling motor setup? I use Trojan SCS200's, so I'm familiar with the weight - 60 lbs each. So you could have 60, 120, or 180 extra lbs, depending on battery setup for your trolling motor. Then factor in the few hundred pounds of equipment that most of us carry - rods, reels, tackle, tool box, anchor, etc....
How do you adjust a "preload" without knowing what the loaded boat will weight? What if the owner is conservative in their loading and doesn't like to put more than half a tank in it because it is faster that way? Then the boat is too light on the trailer and the "preload" causes uneven tire wear on the outside of the tires. Or what if you are like I used to be and want to ensure you have enough gas to go anywhere you want, and you load the boats storage compartments with almost every possible tackle combination? Then you weight much more than the "preload" and you wear out the inside of your tires.
Wouldn't it be better just to get a straight axle that can carry the weight, no matter how you load it?
All the best,
Glenn