Releasing can give both a boost and a drop in zoo rating, if you look at the zoo fame breakdown you'll notice points for number of animals and number of species as well as release to the wild. So unless you have plenty of species and animals you can get a drop in rating if you release the last of a species. Also notice which animal you release, if you release a high-rating one you can take a hit in the exciting animal category.
Just for the record:
To max your fame points you'll need: 20 species, 40 animals and release 10 to the wild. (plus happy animals, happy guests, etc.)