It depends on the risk of people shooting at you and how quickly you can die from them. For example, if you're in a close-ranged duel against most heroes staying still will be nearly an instant loss, so you should definitely strafe.

However, if you're on the highground as Soldier and the enemies can't shoot at you, there is no reason to not maximize your damage by standing still for at least part of your clip.

Depending on the circumstance, you can even compromise between the two by jumping while shooting. This allows you to have a predictable arc to aim with while not being a total sitting duck to all projectiles (though hitscans can still often hit you).