You might find different opinions on what is unnecessary but you could start by looking at your logs, or indexing everything then looking at stats on message numbers then applying a null-queue transform to the messages you want to avoid. Here is a starting point which is dated but links to some newer info:
http://answers.splunk.com/answers/39916/need-help-filtering-cisco-asa-logs-at-index-time
Hope this helps,
Sean