If you feel like you are wasting your time in opening every post page and choosing the same tag again and again, then try this.
TumblrTagCommand for Greasemonkey & Firefox
This script allows you to add a tag to a group of posts at once.
It works inside Dashboard. You don’t have to go back and forth when you tag your existing posts.
The installation may be bothersome, but I’m sure it’s worth.
p. s. when you find something strange, feel free to ask me.