Washing your Pet
Over time, your pet becomes dirty. If you do not wash your pet, he/she will complain often about being dirty. They will interrupt actions to complain and wiggle their hands over their head. Your pet will also (under the right circumstances) poo on the floor. It is completely optional to wash your pet.
Washing pets gets you coins and experience. The more flies a pet has buzzing around their heads, the more money you make. You will get 40 coins and 6 paw points for each fly you clean off a pet. There is no bonus for continually cleaning all flies off at once; you can clean a fly, rest, and clean another fly and still get the same rewards as cleaning it in one go. Cleaning the maximum amount of flies gives 160 coins and 24 paw points.
Some people prefer to leave their pet dirty. There is a thread in the Playfish/Pet Society forum called "Say No to Soap". They have a club of sorts. All the "members" have agreed to leave their pets dirty so their friends can make extra coins.