I loose one of my SMB shares about once a week. Rather than reboot, I just go into setup uncheck the share, exit, go back into setup, and recheck the share. I believe that someone posted a command, which they wrapped a script around, to remount all of the SMB shares. I have NFS setup and never have a problem with loosing them. I do not have the roku on a static IP as others have mentioned.
I also don't have a static IP, but the main reason my Roku lose's the shares on one machine is that I don't leave that machine on 24/7, so every time I fire the machine up again I have to get the share back into the Roku.
I've been using the exact method you described (uncheck, exit, re-check) and it's always worked for me.