Being an exit node and using Tor are two distinct things - you don't become an exit node just by using it (unless you've specifically got software setup that way).
Being an exit node is a good thing though - again, it helps to spread the load. makes it faster, makes traffic analysis harder and so on - and if you have the bandwidth to spare it's easy to setup:
https://www.torproject.org/docs/tor-doc-relay.html.en