I notice that when the master web in my setup does not have the latest patch, it then blocks all access to content on the slave webs.

Is this an absolute must? or Can I "deactivate" this?

Im not sure what the thinking is behind this. When do the sites go down? After some time established protocol from the moment a patch is made available on your server?

How do I know when I have to visit the Control/master web to see if a patch is available and then to patch it before it blocks all my live webs?

Theorically, there is nothing like that in JMS.

You can still use old JMS that are not necessary using the latest version or patches.
As long as there is no specific requirement to install an update of JMS for a specific reason like compatiblity with latest Joomla or get extension definition or get new patches or some extensions, there is no need to proceed with an update of JMS.

What you let me suppose is that you didn't have any patches installed or you got an error while installing the latest patches that may performed an "uninstalled" of all patches (roll-back from the backup).

Normally, you should have any errors in the JMS patches.
The only one that is acceptable is the one that concern the /installation directory that can be missing in the case where you don't need to create fresh slave site.

Well I assure you that all my slave sites are blocked at the front end with a JMS message about improper patches (Ill take a screen capture next time).

When I patch the control web, they return to normal function.

What this means is I have to constantly monitor the control web to make sure it's working. Or wait until my client complains all his webs are down.

Something that you may have to check is the JMS check patches after that you install an update of an extension and also when you update joomla itsef that may restore files without the patches.