Alot of things to take into account when choosing tire size between quals & mains.
Like track surface, how long the main will be & how deep your pockets are
I assume you know there will be tire wear & as the tire wears the ride height of your car changes, making your set up change.
I do not now of any driver out there that runs a 1 hour main, can use 1 set of tires, they useually have to do a tire change during there race.
As for me I useually can get 3 qualifiers with one set of tires. As long as I dont chunk one of them.
I start with 68f and 75r and in between i recheck my ride height, dont need my chassis scraping the ground
I am sure there will be more people willing to adress this topic but this all I can think of right at the moment.