It is not a constant across all search engines with the importance varying according to the tag - engine pair.
For instance IMO the only SEs that Use the KW metatag for any sort of ranking, are MSN,Yahoo and Altavista, but all the search engines use the page title in some way when ranking, and many use the meta description. I do advise using the keywords meta tag since the three engines above use it, but suggest that you do not try to stuff it full of keywords that have nothing to do with that page.
What is fairly constant however it that head tags apply only to the pages they are located on, Most search engines use the Page title as the SERP title and many engines use some or all of the meta description in the SERP description, thus making them doubly important - they are used for ranking and more importantly they are the only sales tool you have to use to convince the searchers who see your high rankings that yours is the page they should go to.
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