It depends on how the table row is selected. I've done various tests and it always seems to add it around the entire TR in my tests. However I do know Dreamweaver can have selection issues and I usually go to code view and make sure the entire <tr> with end tag is selected before applying.
I'm not positive if the issue is with the initial selection or actual application because I've been unable to reproduce it, but If you gave me a copy of the page you experienced this problem with I could take a look and see if I could reproduce it and get it fixed.