The basic rule here is applicable to more than just SEO: better to have it and not need it than need it and not have it. As long as you don't keyword stuff or otherwise spam these tags, you're not going to do yourself any harm, and you might just do some good.

Meta Keywords: no more than 15 terms with most relevant terms first. Don't use terms that don't occur in page. Avoid repetition.

Meta Description: Keep it short, direct, to point, and page specific. No longer then 25 - 30 words. Some engines (Temoa) use meta description as blurb that goes under link. So write an accurate description of page: "All about Joe Schmoe Widgets."

SEM is search engine marketing. Like any marketing function, it involves paying for delivery an advertising message in an attempt to gain customers to make sales or to promote brand awareness. You take how much you spend, figure out how many sales derived from your effort, bump that up against how much you made, and figure out if that campaign was effective. Marketing is typically conducted to promote a service or product that already exists and is immediately available.

SEO doesn't work like that. The best time to optimize your site is pre-production. Do good SEO as you build your site and you won't have to do anything more except a tweak here and there from then on out. It's a lot easier to do SEO up front than go back and redo it later.