This is the question that many keep asking, and to be honest Google has not really been clear about. If you read their webmaster’s guide to SEO, Google states that you should not use unauthorized programs such as Web Position to check your ranking. That is all fine and dandy until you read the whole story behind Google Web API. In essence, you are allowed to use Google API to check your ranking as long as you are making less than a 1000 queries. I personally believe that abusing Web Position could have some effect on your rankings but her are the mind boggling questions:

1. If you are limiting the No. of times people can access your DB, then why would you ban them for putting load on your servers?
2. If my competitors get Web Position and keep running it every few hours, would it get me banned?

As far as using Web Position goes, I think using it once a week or once every two weeks should be fine. But don’t push your luck by checking your rankings every single hour.