[toni-a]

For me it's practically a must have, as said, mainly as fill flah outdoors,but also indoors however I almost never use as direct flash but bounced.

light reflectors are extremely useful also, however you need someone to assist you outdoors they brilliantly replace the flah, indoors for portraits try bouncing the flah light on a golden/silver reflector and enjoy magic.

IMHO there are no do and dont, all depends on what you intend to do and how you are used to this equipment, a wedding isn't the best place to experiment with new gear and techniques, whatever technique you master will,work just fine.