I think this depends a lot on the nature of and venue for the poster. Are you talking about a conference poster that accompanies an article in the proceedings, and the proceedings are published by the time of the conference? If so, then one of the main purposes of your poster is to get people to read your paper. Your paper already has all the relevant references, and so (in most cases) there's no need to waste valuable space repeating them on the poster.
If you are talking about a poster that does not accompany a published paper, then you should use your best judgment about whether references serve a useful purpose. If the material you present has a particularly strong relationship or dependency on past work by yourself or others, and you think your audience might want to consult this past work for themselves, then by all means provide references. To save space, I would make these only as long as necessary to identify the referenced works.
Another possibility might be for you to leave the references off the poster but include them in a handout for interested visitors. At all my poster talks, I print off several A4-size copies of the poster and hand them out to people who wander by to have a look. My posters don't normally include references, but if I thought they were necessary, I'd print them on the reverse of the handout.