I usually tweak stats on the fly for encounters. It takes me less than a minute to jot down stats for a Redhand Goblin Arquebusier, and I can do it in my head if time is an issue. I rarely run encounters without multiple groups/types of enemies, schticks, terrain, interesting 'named' npcs, etc though so to each their own. To be honest I like optimized parties better as I can throw more ridiculous situations at them (fighting a hive of giant psychic red ants on a balloon while a black dragon takes passes at them) (fighting a hag that has teleport-by attack and teleports between ice mirrors that smashing them releases frozen bladerager trolls and other beasties) (hanging onto the body of an elder god and being attacked by it's broodkin as it falls through a tear in the astral plane surrounded by nothingness while a manic group of mindpirates chase them in a lightning-powered flying burrowing machine) and expect them to survive.
Sometimes if an encounter was too weak I pull a ninth hour power-up or sudden reveal kind of thing. Dragonborn ninja drops from the roof, uses Entangling Exhalation on the party, iaijutsu sudden strikes the rogue. Half-Ogre Expertise Fighter suddenly rages, gets +1 size category from that feat, and is power attacking and shock trooping etc etc. Wizard pulls out black onyx ewer and smashes it on the floor, portal opens up and 2 negative energy elementals walk through.
It doesn't take a lot of effort to just add stuff in. Great part is you can just balance on the fly instead of trying to work it out beforehand.