I would always start by building the thing that the set was for, then leave it together for as long as it took to show everyone in the house, and then promptly took it back apart and put the pieces in with what I already had for free-form building. Well, OK, there was usually a delay in that last step, because I was usually at Gramma’s house when I got a new set, and the rest of my pieces were at home, but the set would come home from Gramma’s disassembled.
I kept all of my pieces neatly sorted. I wouldn’t have had any objection to mixing pieces from different themes, but most of my sets were Legoland Space, because I mostly wanted to build spaceships and other science-fiction things, and the space sets tended to have pieces like wings and rocket engines that I’d want.
And of course I never threw away the directions, even though I never used them again. That’s information! You don’t just go throwing away information!