You mean for services.teamviewer.enable = true; ? No, you should not need any reboot: when you set services.teamviewer.enable = true; NixOs is clever enough to start/stop/restart the good services. If you set services.teamviewer.enable = false;, it will completely uninstall teamviewer, so sudo systemctl start teamviewerd.service will not work. You can however disable the daemon if you don’t want it to start by default, I guess:
systemd.services.teamviewerd.wantedBy = lib.mkForce [ ];
should work based on this answer Disable a systemd service while having it in NixOS's conf - #3 by jtojnar