Well, it depends on how long you plan on keeping the wort around before adding the yeast.
Microbes are everywhere- there's no avoiding that. But, if you boil the wort, cool it and then add the yeast, the yeast generally "takes over" before any microbes can. Then once there is alcohol present, it's less likely to get infected. You can keep vodka at room temperature indefinitely- because no microbes can live in it, for example. So, once fermentation is underway, you're fine. Cover the starter with some sanitized foil (to keep fruitflies, etc out) and you'll be fine.