All the major ones, no not really. A lot of the smaller ones, yes. I would add them it couldn't hurt. I wouldn't put too much emphasis on them though.
You'll rank higher in the search engines without it, but thing is, Adwords uses it to determine page quality score.
I've never seen a site in the top 10 really that doesn't use meta tags. Even google recommends using them. They may not hold much weight but they do help a tad.
Nobody knows for sure. I think it's better to be safe than sorry. Besides, keywords are useful for reasons other than SEO.