Multicultural Alien Planet: Well, I try. In practice, it varies by species, depending on how much time I’ve spent detailing them in particular, but they’re all intended to be this way, outside the odd Hive Mind, or other special cases like the Equality Concord.
As another note, this is mostly true of homeworlds, and true of colonies to a much lesser extent. While homeworlds have usually had plenty of time to evolve regional cultural differences in the absence of high-speed communications, later colonies have generally had those homogenizing their cultures from day one. Between colony worlds, though, you can find the same sort of cultural delta.