I have a Joomla 3.4.5 site with JMS and each child site has its own database. My question is about restricting user registration on some sites, but not on others.

As you may be aware, there is a default Joomla User Registration URL that anyone could access directly and submit a new registration. Because my master site is just for managing the various slave sites (no live content on the master), I would like to turn off User Registration in the User Options. If I do that, will I be able to turn it on in selected slave sites?

I assume that since each of my slaves sites has its own database and that users are not shared, that the master being turned off would have no effect on the slaves, and that individual slaves can be set as needed.

Please confirm.

Also, if the JMS site had one shared database, would turning it off in the master admin area turn it off for all slave sites?

Yes you right.
Because you have specific (separated - not shared) users between the different website then yes your analysis is correct.
This is like if you had totally independent joomla and their configuration is specific for each site.
The fact that you use the same DB does not mean that you share the website content as long as you are using different table prefix. Their content remaains separated.

So you did a good analysis. Congratulation.

Thank you!