I clip the winch line onto the dolphin striker for help pulling the boat up, but it depends how you do it.
If I'm retrieving off a ramp then already the back rollers are in (or really close to) the water and therefore there is very little force on the dolphin striker when I'm pulling the boat up.
On the other hand if I'm trying to roll it up from beach wheels or something it's a little steeper angle and I'd only use the winch to hold the boat from rolling backwards while using muscle power to actually get the boat up on the trailer.
OTOH once the boat is up and centered, I unclip the hook and give it a half-turn around the post (behind and then back out the front) and then down and around the trailer crossbar, and clip it to itself. (Did you follow that?) Then I just rachet it snug. That holds the boat firmly to the front crossbar.