I've always been a bit paranoid on the ramp so my procedure is
1. transom straps removed and all other prep done in parking lot, safety chain stays on
2. reverse down ramp until hitch is at water's edge. Unplug trailer electrical connection. De-tension winch strap, safety chain stays on
3. back in until water is just at top of trailer fenders, boat will float free
4. unhook safety chain, pull trailer forward
If it's a ramp I'm familiar with that has a moderate angle, I'll remove the safety chain and winch strap at the water's edge too. I've seen the odd boat dumped onto the concrete on steep ramps, but never the opposite. Surely you'd be waaay too far back if the truck rear end floated off??