If you turn off ClamAV and it was already running SmarterMail will not kill it. Same could be said if it were previously running and you rebooted SM, the ClamAV is never stopped by SmarterMail except when we do a "freshclam" update.
However, restarting your service even if ClamAV was not restarted should still function. SmarterMail would see it's already running and try to connect to it. So to answer your question you shouldn't have to worry about rebooting ClamAV when restarting SmarterMail, you will lose communication with ClamAV but on boot it should reconnect. It will only do this reconnection when it goes to scan an email, so if you don't see it immediately reconnect it could be that it has not yet started processing a message.