I might not have stats right when the fight starts but when it does and I have to roll, I write down their stats when it comes up. Kind of like a villainous version of on-the-fly character creation.
I prefer, however, to have stats ready in advance. If I do have time for stats, I try to make sure they have meaningful ways to interact with the PCs as well as the stats to back up whatever evil machinations they'd be doing if there WERE no PCs to interfere. My group may not attempt to damage a villain's Resources, but if he's set up as a corrupt CEO trying to buy a congressperson, he'd BETTER have something in his Resources stat.