Just my opinions based on two totally different Yamaha boats on three different trailers...
Why no winching for me... it's a really heavy boat and breaking that winch strap on a ramp means you're fairly well hosed. That's why I try to winch as little as possible and I use Liquid Roller on the front bunks. If I can power up gently, I do. If not, I get the front bunks to where they're holding the boat and winch up as much as I can. If I can't get all the way without serious effort, I'll make sure the bow is over the roller and back down an inch or two and try again. Too deep and the rear won't engage, which gets you a crooked boat on the back bunks. Trailer guides help this issue immensely and I'm probably going that route soon. General observation: Boats with the front "square nose" (mostly 21' and 24' - usually ones with a ladder) are much harder to get right.
I never had a problem with my LX... Back in to any depth, float off, drive on, was dirt simple. The 212X is a hassle and has been since day one. The boat hangs on the middle of the front bunks no matter what I do, how deep it is or the ramp angle. Without Liquid Roller or lots of power, it's just not going on. My friend's SX240 is similar and hits the trailer the same way, but doesn't hang up as much. Of course, it weighs way more and doesn't like being winched at all.
As for straps, when unloading, I leave the safety hook through the bow eye and will loosen the winch strap slightly, but leave it engaged and ratcheting so it won't spool out. I've never had the boat slide back until it's well in the water, even with the Liquid Roller. Loosening the winch strap slightly lets the back end float without pulling the strap or digging the nose into the roller, but still keeps it attached to the trailer should a wake, wind or sudden stop try to get it free.