This has been discussed / debated at length here in previous threads.
In short, several good anecdotes were told that supported the theory that alternators don't like to be turned on when spinning. Countervailing anecdotes were also provided.
I think the wise approach is to always turn on both sides of the master switch for a "normal" start. This allows the alternator to spool up its output as it comes up to rotational speed following engine start. If, on the other hand, you are in an "abnormal" condition where every last available coulomb of juice is required to just barely turn over the engine, turn on the alternator only after that very dicey start has been accomplished, understanding that by doing so there is more than a tiny risk that the sudden turn-on of the alternator may produce a puff of expensive smoke.
In the case of our aircraft with a B&C alternator and B&C linear regulator this process works quite well.
As always, your mileage may vary, don't try this at home, filmed using trained stunt dummies, etc etc etc.