Mysql Master Offline
If the master crashes, then it doesn't really matter what you do as far as stopping the slave, as long as you observe the slave once the master is back online, to verify that it has successfully started reading and executing events from the master again.

If you don't stop it, the slave should still be fine, and will sit and continually try to reconnect to the master based on master-retry-count and slave_net_timeout.  These retries are not going to have any significant performance impact on other queries running on the slave.  After enough retries, the slave will eventually give up, after which STOP SLAVE; followed by START SLAVE; should bring it back online again (or make it try again for the allocated number of retries).

If you take the master down for maintenance, the safest thing to do would arguably be to STOP SLAVE IO_THREAD; on the slave before shutting down the master, but if you don't do this, the master should shut down the slave's connection gracefully.  The slave will still continue to retry, but the retries as the slave attempts to reconnect aren't going to do any harm.

http://dev.mysql.com/doc/refman/5.6/en/replication-options-slave.html