If the users of your website should really get the poll just once, then yes. I don't know a better way to make sure they haven't already answered the question.
If it doesn't matter that much, you might wanna use just cookies. So, those with cookies on will get it only once. The others will get it everytime. But then, ofcourse, you have no control over the score, so usually this isn't acceptable.
I know, it's a bit of a tough choice. Alas, there are always ways to cheat on polls. :-(
Gtz,
Kristof