I've never found a need to do this. I run my motor on muffs and top off the battery charge the day before I take it out. Then start it on the trailer after backing in (water intake submerged) and power off the trailer. From what I've seen, pretty standard practice here at the San Diego and Mission Bay launch ramps (concrete surface, so no issue with power loading or launching).
I do, however, occasionally see guys firing up there motors up in the parking lot and letting them run for as long as a minute. Doesn't seem like a good idea to me.