It should not make one iota of difference.
The current draw of an alternator is less than that of a permanently on beacon, and you can add to that load the gyro's on the electrically driven instruments.
The current required for starting is so large, that even the manufacturers recognise this fact by not enabling you to isolate such circuits and permit all allowable amperage to be delivered only for cranking purposes. I.e the draw is so low, that if it won't crank with them on, it isn't going to crank with them off either.
qouting plovett's engineer.
One chief engineer reckoned the battery was the cheapest part of the engine start so don't risk damaging the alternators until after the engines are turning.
Hmm...my experience is an internal regulator is 1/4 the price of an aviation approved battery.
I'm pretty sure said engineer is rather uneducated on electrics, or just living on OWT's (possibly dating back to generators).
If you consider that most Aviation alternators are an 'approved' version, if not identical to one used in automotive, then things become a little more relevant. For example, a 1967 Cherokee 6 shares the same alternator as a Chrysler of the same era (no not tell you the model). The only difference is that one has an aviation serial number/approval on it.
This leads us to the fact that there is NO fundamental difference between automotive and aviation alternators. They work the same, act the same, generate the same result.
If you turn the alt on before starting, how is this going to damage the alternator? If the risk of damage to the alternator is real, then why is it that NO automotive manufacturer configures their electrical system to switch off the alternator during cranking? Afterall, with their warranties, they would want to spend the 50 cents on a solenoid/relay to do just that, if it was a problem. They don't spend that money, and the power to your cars alternator is permanently live!. don't beleive me...grab a test light and go check for yourself......its the BIG wire on the back of the alternator...compare your car to your aeroplane.
The only benefit of us being able to turn the alternator off, is when we see a problem...i.e overcharge situation (regulator failure), and we can start a process of protecting systems and our lives. In a car, it'll charge until something fails/burns. Usually battery first.