sqlnet.ora or tnsnames.ora changes require reboot?
In general, no.  A tnsnames.ora change shouldn't require a reboot but some applications will read and parse the tnsnames.ora at startup to be able to present a drop-down list of servers to the user, for example, and will cache whatever was read when the application started up rather than re-reading the file.  Depending on the situation, it might be easiest to reboot a Windows client rather than figuring out how to kill and restart any applications that might have the data cached.

A sqlnet.ora change might benefit from a reboot for the sake of consistency though it is not required.  If you are doing something like enabling dead connection detection on a server by setting sqlnet.expire_time, for example, it probably makes sense to reboot the server to make sure that the setting applies to all connections rather than just new connections-- if you're trying to debug why a particular dead connection is still hanging around, knowing that the server was restarted and that you're not looking at some artifact of a connection that was opened prior to the setting being made would generally be helpful.