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'Battery cutoff switch' and 'blown alternator diodes' in the same sentence rang a bell in the increasingly fuzzy recesses of my head. So, I've done some googling and found this on a British car forum (where else?).
Basically, using a cutoff switch to stop a running engine is nonsense up with which alternator diodes will not put.
Of course, you knew that and would never do that, but, you know, sometimes we get forgetful in our dotage.
If it matters, or is at all relevant, I've never managed to blow out diodes on an alternator. Too sheltered a life, probably.
"The regulator's job is to maintain about 13.5V at your battery +. If your alternator is turning disconnected from the battery, the regulator "sees" nothing, which is equivalent for him to Ov. So it gives more field to the rotor, to increase the output voltage which goes to the rectifier diodes. Voltage can go up to 80ish V, enough to "burn" the diodes. If you use a master switch without a resistor (or uncorrectly wired resistor), that's what happens if you cut your engine with the switch, especially at high revs (emergency, ...) On well built switches, the resistor is between the output and the earth. Low resistance -> high current -> low voltage -> safe diodes. Jeff LHD 1990 BDR"
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