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On engines equipped with only one impulse coupling, you should always start with only the impulse-coupled mag enabled. Note that on many aircraft, turning the key to "start" does this automatically.
The reason is that a non-impulse-coupled mag (normally the right mag), if it fires, it will fire far too early causing the engine to try and turn backwards (the reason this isn't a problem once the engine is running is that the engine is turning fast enough to catch the combustion event just right if the mag starts it before the piston reaches top-dead-center).
Faster cranking speed will just make it more likely that the non-impulse-coupled mag will fire, which it will do too early.
So, the procedure should normally be: mags on left, mash the starter button, after you release the starter button, mags on both.
Of course, if you have impulse couplings on both mags (like my Tiger), you can start on 'both'. If you're not sure what you have, it's not hard to tell on most Lycomings. Look at the back of the engine where the mags are bolted on to the accessory case. If the mag has an impulse coupling, there'll be about a 1/2" spacer between the base of the mag and the accessory case; if it doesn't have one, the mag will be right against the accessory case.