Current WP version have pretty decent SEO support. IMHO you don’t need any external SEO plugins (except something to generate Google Sitemap maybe), yet you should pay close attention to two things:
1. take a great care of the content – optimize it, learn to write good titles and headings (with SEO AND readers in mind), give some meaningful names (I mean titles as well as filenames) to your images and so on; but be careful not to “overoptimize”
2. check out the theme that you use – is it SEO Friendly? Does it validate (W3C) without any errors? Does it make a proper use of h1,h2… tags and paragraphs (<p>)? The images and links – are they properly formatted (“alt” and “title” attributes)?
That being said, if you take a good care about these things you should go well without any seo plugins ??