Service charges are not compulsory and you can decline if you feel the service wasn't worth it.
But, to answer the question, if it's included I don't tip as well - unless the waiter/waitress is exceptional.
The service was good. But not being used to paying a service charge I wasn't quite sure what the "done thing" was.
We were celebrating my Son's graduation.
The waitress said that she had deducted the price of his Cheesecake dessert off of the bill as a "well done" gesture. But on checking the bill when we got home, we'd been charged for it.
Minor thing, but it was more the tip situation I was concerned about.
My understanding is that it replaces the tip and may be distributed equally amongst all the staff, not just the waiter/waitress who served you. Restaurants have been criticised in the past for creaming a percentage off the top so I don't know if that practice still goes on.