Before you take the boat down to the ramp, make sure you can loosen all the nuts on the u-bolts, unless it's a brand new trailer, I'd spray them with PB Blaster for a couple of days, first, to dissolve any rust. If they are old and rusty, plan on replacing some because it's easy to break them. I rebuilt my old 94 Load Rite and replaced every u-bolt, the axle, springs, spring mounts, added brakes, put on a longer tongue, and longer roller bars. Should have bought a new trailer! But learned a lot. I rinse the trailer in water after each load/launch but 2 x a year, I spray all the u bolts with Corrosion X, and the ones I replaced 6 years ago, have no rust on them still.
I prefer roller trailers, we have a short ramp here and at low tide you could never use a bunk trailer.
If its a used trailer as noted above carefully check the wheel bearings, make sure they are smooth running and have clean grease in them....