That's not the point. For one thing 1/4 turn on a well tensioned wheel may not move it very far, because you are increasing overall tension, and therefore the resistance to lateral changes. Secondly, if you tension first the spokes will be harder to turn, especially as you reach the end of the process, which makes it more likely that you will round off spoke nipples. If the tension is on the low end of acceptable I can see combining tension and dish - it's something mechanics do often. Otherwise as I said it's better practice to loosen one side and then tighten the other.