Permalink Sturctures in wordpress are very important when it comes to SEO for your blog. Usually the best permalink structure most people use is

/%postname%/

as it gives maximum exposure to the keywords in the page. Also some people usually add up .html, .php, .asp to fake the page type to google search bots. This however is not very important. But if you have the right permalink structure, your SERPs will increase for the better. So either use the default postname or add month to it too, but the lesser it is, the better