Steam’s resource footprint is minuscule. Having it running in the background will have no noticeable effect on your gaming or productivity experience on your PC.
Even if you don’t care about friend’s lists and screenshot/video streaming management, and the other bells and whistles that Steam offers, you probably want your game to be fully updated without having it interfere with your gaming time, so background updates to your game are likely something you’ll appreciate having Seam running in the background for. Cloud saves are another biggie for me. I no longer have to worry about backing up save games, I can reinstall an old game and continue where I left off.
You can turn all of these features off if you want to though, and if the game supports it, you can even lunch it without steam. If the game does require Steam to run, you can always exit Steam after running the game, no biggie. Steam does not auto-load on windows startup by default, just make sure you don’t check that setting when installing and you only have to see it when booting up the game.
But really, Steam is good peoples. There’s no need to be worried about it crippling your performance or anything.