The ideal is to have the weight of the boat *almost* fully supported on plenty of centerline rollers, with the bunk boards just kissing the bottom to keep the boat from rocking. To launch, pour a bucket of water on each bunk. Then put the trailer on the inclined ramp and a shove rolls the boat off and into the water.
Unfortunately, most trailers (including mine) are far short of ideal. There are too few centerline rollers, and they lack strength. This has led to two solutions (?) One is to replace the bunk boards with a series of rollers so they can carry more weight without sticking. The other is the Float-On trailer that is actually designed to be submerged. If you use one of these, be sure to keep the trailer bearings greased and watch the "waterproof: tail lights. Good Luck!