Google seo starter guide explains url structure, look here: http://www.google.com/webmasters/docs/search-engine-optimization-starter-guide.pdf
Try to make the URL structure visitor friendly just by keeping one thing that it shouldn't be so long and also it is suggested not to include _ in the URL rather you should use - (hyphen) in the url. try to keep more number of URLs in the website static as many number of dynamic urls are not search engine friendly.
Simple, just use readable URLs. So no ?id=3 but instead this-is-the-title-of-my-article. Nothing else matters really. And it isn't as much about SEO, but also about usability and allowing the user to see an informative URL when he/she does a google search.
URL structure must be Search Engine friendly. No underscore, neither strange characters in your keywords. Just hyphen will do the work esier...