Rule of thumb on what changes require a database reboot I am working on enabling SSL for database listeners. As a part of it, I have to modify the sqlnet.ora and listener.ora files. Does this require a database bouncing to take effect? Is there a general rule of thumb that I can follow when deciding whether or not to reboot the database for some changes to take place.
No reboot of the database is required vis a vis the listener.  The only communication from the db to the listener is for the database to register itself with the listener, which it does every several seconds, on its own. Changes to db init parms only require a reboot if the change was 'scope=pfile', and that is only necessary if the description of the parm (in the Database Reference) says 'modifiable - NO'. 

The db and the listener are not as tightly linked as you seem to think (based on this and other questions you've asked).  They are separate and distinct processes, and the listener's only role is to receive connection requests, and set them up by spawning a dedicated server process or connecting the client to a dispatcher process.  Once that is done the listener is out of the picture and you can even stop it without impacting existing connections.