I will typically keep enough for one standard award for use in an emergency. (Real or perceived)

I do like to save for nice trips. At Christmas, I flew my three grown children to meet us in Hawaii. I'm a hero and it didn't cost anything.

Some people will also travel with the primary flyer buying the ticket and accumulating the miles (and bonuses) and the others taking the freebies.

There is no right answer...it's personal preference.