Compared to wheels and tires, valve stem caps are a funny little side note. They're easy to lose, easy to buy and easy to forget about completely. And because they're so easily interchangeable and relatively inexpensive, buying custom alien, skull or fairy princess stem caps is a tempting way to put your personality on display.
But stem caps fulfill an important function: keeping air in your tires. Without them, even a new stem tends to ever so slowly bleed air. And a low-pressure tire is a squirrelly tire, prone to pinch flats and low gas mileage.
So make sure your new, flaming-snake stem caps include a little rubber gasket on the inside that will press against the top of your stem, keeping dirt and grime out and air in.
Yeah, it's a picky point, but it's an important one.