I don't think of join up as a magical treatme, or as a tool that is necessary on a well-trained, respectful horses, so IMO if your mare keeps out of your bubble, isn't hard to catch, and responds to you- there isn't a reason to introduce something new to her. I know quite a few people who use joinup as a sort of lunging exercise before riding their horses, and although I do admit that their animals are very well behaved and responsive, joinup to them is just another thing. They are simply reacting the way that they were trained to when running in circles, not using their instincts or their heads to figure something out. I've even had one of said person's geldings attempt to joinup with me when I was lunging him.
That being said, I am pro-joinup under the right circumstances, though. I've used the method with my ill-tempered (and extremely dominant/marish) filly two or three times as a way to establish dominance. I otherwise just roundpen her, which, ofcourse- is quite different.
I advise in just not attempting join up with her, especially if you do not have a friend or trainer who can teach you the first time- as I have heard of many stories (often on this forum) from people who attempt to join up with their long-time horses and end up with a big mess (such as a charging, rearing, or kicking horse)
If it ain't broke, don't fix it! ^_^