As others have said, no specified angle. At least none that I know of.
But, something to consider. When your towing the boat out of a driveway or parking lot that drops down onto (is higher than) the road, the skeg needs be high enough so that it will not contact the ground once the trailer wheels have reached the road surface.
As a rule, I like the bottom tip of the skeg higher than the rear most trailer cross member.
Two other notes. Some Yamaha dealerships are telling owners it's ok to trailer using the tilt lever on the motor. The manual says otherwise. At least in the case of my '03 F225 it says otherwise.
When using the M-Y wedge, for some motors it matters which side you use it on. In the case of my F225, it should NOT be used on the side where the zerk fitting is located.