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1. the keyword metatag has not been a part of the Google algo for some time
2. the description metatag is used to compose snippets - but again, it's not apparently used for ranking purposes at present.
Because of this, I try to write a good description - one that "attracts the click" - on the first shot. I only change the description if something looks really wonky on a SERP. This can happen if a page starts to rank and drive traffic for a search that I didn't plan for. If that keyword or phrase isn't included in the meta description, then Google might assemble the snippet from other ingredients. In that case I may re-write the description to gain some control of how the result looks to the user.
If you're concern is improved ranking, frequent changes to meta tags are not the best place to spend your energies, IMO.
Because of that thinking, I follow Tedster's advice as much as possible -- write one time and then leave well enough alone.
Tomorrow it will be tagged as item 2 and the day after item 3. Item 1 will have a new title, description, and content.
That is a problem, but one that most CMS and forum designers have not yet woken up to.
I wonder -- and I'm speculating here -- if the search engines (especially Google) get suspicious of websites that change their descriptions and/or titles too often, thinking perhaps that it's an attempt to manipulate the algo.
Google does collect your history and reviews it. If I were a search engine, what would I think?
Many changes of tags are uncommon, unnatural, and unjustified, especially without corresponding significant on-page content changes. Must be an evil optimizer fiddling. (Who else does that?)
If title tweaking is bad, what about 301 to a page with a new/different title?
On one of our e-commerce sites with about 2000 products, we're planning on implementing a more SEO friendly url and part of that process is to make a "better" title. Would it be better to get all the titles re-done at once, or gradually work them into the site? And, ideally, should the title be updated before the SEO friendly url is implemented, or at the same time, or after implementing SEO friendly urls?