Is it the cake or the frosting? Attend any birthday party and you'll hear that debate. In the end, frosting always wins. With Search Engine Optimization (SEO), I place my bets on the cake. While it's true you can fix a lopsided cake by piling on the frosting, wouldn't be better to start with a well baked cake?
Addressing your SEO while composing just makes good sense. For WordPress users, there's a plugin for that. WordPress SEO by Yoast adds functionality to the compose screen. It allows you to add or edit each of the essiental SEO elements while you're writing instead of after. I start by choosing a focus keyword, then the description, then editing the SEO Title. Saving a draft updates the SEO review allowing you to tweek the post and the meta tags, before you publish and send it off to the search engines for review.