To be honest, I often colonize the big breathable at the same time (or earlier) than the SY planet. But I never waste time on building a SY (or space port) first on a BB, always going straight for resource or research facilities (only exception would be a small minefield).
Small Example:
Huge breathable wanted for research. You have advanced storage tech and decent research bonus (lets use 700 points pr research facility). Difference between building 29 Researh facilities first and then the shipyard and building SY first and then the research facilities (Total buildtime 34 turns).
Difference in research points produced by turn 34: 101 500 Points
Every turn wasted on building defences, colonizers or whatever before starting the facilities: Another 20 300 points
down the drain.