The short answer is ALWAYS!  But let’s rewind a bit and provide some context.

You probably know by now that IE8 renders pages in standard mode by default.  The IE8 team made this decision to ensure the best experience for their users moving forward.  At the same time, the team has been very concerned about all the “hacks” that people had to go through to render pages using older versions of IE.  This is why Microsoft decided to provide a compatibility mode in IE8 to help during the transition and provide the best possible experience to users of non standard web sites.

A very complete summary on the IE8 team blog about compatibility can be found following this link. 

Here is an excerpt from the above post that really answers the question: “to tag or not to tag?”

“Site owners are *always* in control of their content. By default, Internet Explorer uses DOCTYPE switching to determine Quirks v. Standards mode (again, Standards mode maps to IE8 Standards by default). Site owners can choose to use the X-UA-Compatible tag to be absolutely declarative about how they’d like their site to display and to map Standards mode pages to IE7 Standards. Use of the X-UA-Compatible tag overrides Compatibility View on the client.” 

Compatibility mode can be triggered programmatically and I believe this is what every developer should do.  The reason is simple, you want to give your customer the best possible experience on your site.  You don't need to give your customer unnecessary choices.

There are several scenarios:

- If you have already tested your web site in IE7 and you are not yet prepared to make the transition you can just add the compatibility meta tag to all  your pages and they will automatically render in IE7 mode.

- If you are gradually transitioning to more standard code, you can selectively add the compatibility meta tag to the pages that you have not fixed and maybe never will (outdated content, support pages, etc...).

- If your code works well with IE8 then you should add the IE8 met tag!  This will ensure that your code will continue to render correctly even after the next browser transition.

In summary, the compatibility UA-X meta tag is the perfect way to ensure that your customers will enjoy your pages the way they were intended.

At ArtinSoft we have created the IE8 Compatibility Wizard that allows you to automate the meta tagging process.  You can check it out at: http://www.aggiorno.com/aggiornoexpress.aspx

Go ahead and TAG all your pages!