to play safe..
its adviced to have the meta set at the site template... and so it can appear in every pages.
usually i will do that.
Meta tags still carry some weight especially the description tag which will be used in Google unless you have a DMOZ entry in which case the beginning of the description on there will be used. This in turn can be switched off with a special meta tag in your site.
Charles is right. The Meta tag should introduce to the content of your page body. I.e. meta keywords that are just in the header and nowhere to be found in the page body are not much use.
there is too many search engines.. and some are meta search engines.
where meta tag play important roles as well.
google and yahoo not work the same.
so, meta is a good help for them, some how assist their bots to do the right thing.