Being dirty is fun. It carries a negative connotation only because of old religious notions of being unclean if you have sex before marriage. Now, the word "filthy" is a good thing.
But as with all things, there can be too much of a good thing. Honestly, the perfect amount of filthy is whatever level you're comfortable with.
If you're unsure, here's a helpful guideline to give you a general touchstone to perfectly acceptable levels of filth.
