In the US, there are poisonous spiders in all the lower 48 states. The black widow and yellow sack spider are throughout the US. The brown recluse and hobo spiders also have a wide range. All will mess you up if they bite you.
Link to a map of poisonous spider distribution in the US.
Sure, most spiders are safe, but can you
identify a yellow sack spider? Squish them just in case.