Is it appropriate to attend a class one is not registered for?

Is it appropriate for a student to attend a class s/he is not registered for?

I am a CS student and I am almost done with my degree, but there are some very interesting electives that I will not get to take and I would love to simply attend those courses any way.

Is this inappropriate to do? Should I ask the professor's permission first?

I am not sure what my school's policy on the matter is, since I could not find a single source on the topic online from them.