I wouldn't expect to... I've been using a pebble watch for almost two years now and it makes very little, if any impact on the battery life. Additionally, the phone's screen is one of the biggest power users, so if using the watch allows you to keep the phone screen off a little more then it will only help increase battery life.
I don't really think that you'll see an increase in phone battery life when you start using the apple watch, but I really don't think you'll see a decrease either.