One thing you have to understand is that Google is always evolving for the better. You didn’t think they would let you do 1,000 directory submissions forever did you? Google is in the process of making some very interesting algorithm changes and for some of you these changes might it hard. In the not so distant future too much SEO could trigger a penalty in the eyes of Google. What is too much SEO? Well that will be up to you to determine. If it is construed as manipulative in anyway it might trigger a red flag.
Here is a great video from Rand Fishkin where he discusses some very important areas to review with your website and your external search engine optimization approach to make sure you do not end up on Google’s bad list. If it seems like it would be an approach that Google might not take a liking to you should avoid it all costs.