To do otherwise means you could be taking off with a tank selected, whose delivery has not been verified.
You can't guarantee to verify both tanks on the ground and switching just before take-off makes no sense. Better to find out that a tank isn't supplying at 2000 feet in the cruise than at 200 feet on take-off.
Here at Eggesford I could be airborne in under a minute from start-up and my Lycoming manual discourages ground-running for more than 4 minutes.
QDM