Can any of you snake whackers answer this?

No, you do not get credit, but you also don't lose any snakes. You can whack 5 or 10 snakes in each friends town each day. It doesn't affect their snake counts. Just be careful because there is a chance you will set some of your snakes free anytime you enter a friends town. Check your count before you enter their town and immediately once you've entered. If your count is down then you have released more snakes in their town. You will be able to whack more than the 5 to recatch these snakes.