It never seems like a good idea to tie up the boat launch for an extra minutes to blow out the engine compartment for something that I have never heard of actually happening.
it happen, years ago, to a 19' inboard runabout. I just happened to be looking right at it, when the boat disappeared in an instant, leaving behind only a shower of splinters falling from the sky, and the two occupants badly injured and in the water.
Nevertheless, IMO, in a true emergency, you should do what you must
to respond to the demands of the occasion, and you should balance all the risks in your mind. How great is the risk that the boat will explode if you start the gas engine without running the blower, as compared with the risk that the MOB will be lost or suffer hypothermia or other danger if you don't use the engine to recover them quickly? If you can
maneuver the boat under sail sufficiently to recover the MOB, then that's what you should do. If you must
use the engine, are the circumstances such that you can do a quick sniff test and run the blower for a minute? If not, then your only acceptable alternative might be to start the motor and cross your fingers. There is a known risk in starting a gas engine without checking for fumes and using the blower, but, that doesn't mean that it is likely
that a disaster will follow. If the alternative is that you won't be able to recover the MOB, then that might be a risk that is reasonable for you to take. IMO, you do what you must do under the circumstances.
There might be an alternative to tying up the launch ramp while you run the blower. For example, you might be able to turn on the blower and let it run while you are unstrapping the boat from the trailer and launching it, or perhaps after you launch it, while someone is parking the trailer. You can often find a convenient time to run the blower, if you give it some thought. I'm sure a lot of folks run the blower before they start the engine. Watch how they do it and learn from them.