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One last note here, you are speaking about publicly traded companies with Nike, Best Buy, etc... These companies have a dedicated staff that work on the site all the time.  They are not going to put on the front page of their site, last updated by Robert Smith.  I also agree about the fee, I usually deduct it from the cost of the site design similar to how construction companies give you a discount for putting a sign in your yard.  You can also use the Author meta tag.

The "tagline" as you put it is not really a tagline but we will use it.  It may not be seen by users but Google will see it.

Design is design, I guess you feel your work is not as important as an artist, not my problem.  Someone doesn't want my links they don't get it but usually after design they want to brag of my work to their friends.  But us younger folks use the internet for our reputation not the newpaper.

It is a backlink so yes, it is helpful, in the footer in my opinion very helpful.  Then It started off as being helpful to add html links to the bottom of the page so when listed with Google, Google would include the pages in the search results, now called Sitelinks.  This has evolved and if you use a CMS like Joomla or Wordpress can easily be distinguished without them.  It is appropriate to allow the company working on your site to include designed by....  I will use it to boost the keyword I want.  For example say I want to rank higher for Indianapolis Website Design over the next 3 months, I will start including it and link to the site design page of my site.

I started out as a Windows guy using IIS at which point you want to setup the primary domain account then setup the dns alias to point to the primary domain.  This way there is no marketing involved because they point to the same location.  When setting up a webmaster account I use the www but it really doesn't matter because it still points to the original domain.