Quick WordPress Tip about deleted Tags Creating 404 errors in Google Webmaster Tools
If you delete a wordpress tag, it will create a 404 error if a visitor or possibly as important, Google Bot comes through to re-index your site.
Sure you can redirect that deleted tag link, but if you later create it again on the fly with a new post, your redirection will drive people away from the tag. 
Solution – Don’t Delete WordPress Tags!
If you have already deleted a WordPress Tag, think about adding it back in again so that you don’t get nicked by the Search Engines! 
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got a lot of tags and was planning to clean up some of them. then i read this post. thanks for sharing! i guess it would not hurt that much to just leave those tags alone anyways