I use an internally regulated alternator, & don't make any attempt to take it off line during normal operations. For normal startup sequence, I never worry about it, and wouldn't do it if I had an externally regulated alternator, either.
With an externally regulated alternator, it's good to have control of the field (pullable breaker or switch) for those times that you need to sit in the a/c & perform some task that requires electrical power, but without a need to crank the engine. Reason is that many external regulators will bring the field winding on line any time the bus is hot, in an attempt to get alternator output up to the set point (not happening when the engine's not running). So you'll have a needless additional 2A-5A of drain on the battery if you don't shut off the field.
Charlie
It's worth mentioning that some older designs (which are still in common use in homebuilts) can have their regulator circuits destroyed if they're shut down while under load. When the load was(is) dumped, those early designs can't catch the field voltage fast enough to prevent a large internal voltage spike, which exceeds the voltage rating of some of the semiconductors in the regulator. Guys were checking their low voltage detector circuits by shutting down the alternator in flight, and killing the alternator.