If it were a valuable project, I would, and I'd also set up traffic reports running off the IIS log file. 

For the average project I'd only redirect the high-priority and/or top-traffic URLs.

Yeah, I doubt there's a module out there that scans the IIS logs.  Not that it couldn't be made, I've just never heard of such a thing.  The thing is that 404 errors are generated by IIS before DNN is even involved, so there's no record in the DNN logs.

Now, you could do something like configure IIS to invoke default.aspx when a 404 is returned, and then handle the redirects with something in DNN.  I've never explored that option, but I'm sure it could be made to work.  I've never had to do more than a few dozen redirects, which are so quick and easy to do it's never been worthwhile for me to explore those advanced possibilities.

The reason that won't work --- at least not without tweaking IIS in the manner I described in the earlier post --- is that true 404 errors are generated by IIS before it "knows" to invoke the web application.  So, for example, if you go to www.yourwebsite.com/example.yyy settings in web.config don't matter because IIS doesn't know .yyy files are ASP.net applications.  If you first do the change I mentioned, tweaking IIS so that 404's bring up an asp.net file then there are all sorts of things you can do, ranging from what Flatburger suggests, to working with SiteUrls.config.  The trick is taking that step so that IIS knows what to do with true 404 errors.

In case I'm not making the point clear, here's another example.  Say you have a new portal and it has a tab at this URL:  http://www.mysite.com/default.aspx?tabid=53  Say that the old site had an equivalent page at http://www.mysite.com/something.htm  Your objective is to have incoming requests for something.htm map to that new URL.  .htm files are not files which invoke asp.net and therefore nothing you do in web.config, or DotNetNuke, will do the redirect.  So either you must make IIS do the redirect (if you don't have many to do the virtual directories I suggested are very easy), or you have to configure IIS so that it invokes asp.net from a true 404 error.  That's the tweak I was telling you about.  As long as you do that, then you can do all sorts of things with DNN, web.config, etc to redirect.

To reiterate in one more way, having an unknown tabid is not the same thing as a 404 from the perspective of the web server.  To DNN --- to the web *application* --- it may be an unknown page.  But that's a different issue than handling a true 404.