From the reddit Wiki:

Please do:

>consider making your bot comment only when specifically called
>
>look around to see if your bot is a duplicate
>
>make sure your bot is actually adding something to the conversation it’s posting in. A bot which says “Good post!” is pointless.
>
>consider having your bot only make top-level replies
>
>have your username or a dedicated subreddit listed in your comments for easy communication
>
>consider giving users a way to opt-out or making it opt-in completely, alternatively offer blacklist options per user and per sub

Please don’t:

>make a bot that deliberately copies comments or posts that the original user may wish to delete at a later date