lucy24 - 12:34 am on Aug 21, 2013 (gmt 0)
In practice you're talking about type-ins. Most of them will just ask for the root, so it makes no difference. But suppose someone puts up a link to a page on your site. A year later, they're cleaning up their page. "dot net? That can't be right, I must have meant dot com" and change it without checking. Not awfully likely-- but it can't do any harm to redirect appropriately.
If people end up on a nonexistent page, let it be a nonexistent page on your real domain where they can benefit from your custom 404 page.
All this is assuming that requests for the other domains still pass through your ordinary htaccess-- or IIS equivalent-- so unwanted robots get slapped with the appropriate 403.