On the Tags page (get there from the Reports menu), you can see a list of all of the tags in your system. You also have the option of clicking the tag to bring up all of the cases using that tag.
However, when I click on the "IT_Maintenance" tag, I also get all of the cases with the "StabilityLab" tag. This is because both of them have "it" in them.
I've read http://fogbugz.stackexchange.com/questions/1136/how-do-i-search-for-an-exact-match and I get the reasoning for why searches behave this way. However, I think that the tag selection functionality should show you only the cases tagged with what was clicked. I don't think it makes sense for it to do anything other than that.
Is there a way have the tag link automatically use the index of the field as described by adambox in the link above?