Assuming that the service is running and in use, and you didn't change the contract or anything - just some underlying logic. After publishing the service to IIS, do you have to restart IIS?
(If it matters, the InstanceContextMode is Single)
Are there any instances where you would/would not need to?
No. You don't need to restart IIS. What will happen is that ASP.NET will notice that the files have been modified and the AppDomain will be reloaded. If you have other web sites hosted on IIS - they won't be affected at all.
It is still recommended that you use a separate app pool for each web site.