There are 2 main google update. One is the PageRank update and the other are algorithmic updates. During a PageRank update, google just computes and recomputes for the PageRank. There is not much change in the factors that google uses for ranking except for the PR.

My belief is that, you can do any SEO optimization during a pagerank update. So since google is doing a PR update now, you can do any whitehat SEO and you will benefit from it. The best way of course is to build more links.

During algorithmic updates however, I don't suggest doing any on-page seo work. Once the updates are finished, you can then check out the top 10 positions in your chosen keyword and observe what type of websites rank in the top 10.

Make special note about the title, meta tags, heading tags, keyword densities, number of outbound links, and other on page factors. Based on your observations from the top 10, apply any necessary change to your website.