So my stop command stems from what inadvertently came out of my mouth the first time I had Nyx offleash, she was 5 months old. We stopped at a lake that had a huge grassy area around it and not another soul in site, so it seemed an ideal time to see how she'd do. Things were going very well until she spooked up a couple quail. They took off and flew just over the tops of the grass, and of course Nyx was in hot pursuit! Fearing that she'd run off too far I literally SCREAMED STOP, then recalled her. Though she'd never been told to stop before, I'm sure the maniacal tone in my voice at the time played a role, and she recalled beautifully. Thinking about this on the way home, I realized how important some command like this would be in keeping her safe down the road. So I started trying to formalize it. I began by walking around our front yard with her on leash and having her "stop" at the boundary. She's really good at this now, as I also have a garden I didn't want her in, so I put up a ribbon barrier and worked on stopping there as well. So, in essence it is a stop-stand command. Now though, as I've tried to get her to stop while coming toward me, she does eventually, but not immediately, which made me think of utilizing a tether initially to convey the intention of immediate response. I was thinking once she understood that at a walking pace, it would easily extend to faster gates with reinforcement without the tether. I've also been teaching her to sit, then lay, then stand in place. She does all these things well, except for the stand...she always advances toward me a few steps. So again I thought perhaps using a tether and having her go through these different positions would help her understand that "stand" does not include walking forward.
Anyway, I'll incorporate the techniques mentioned by Steve above, I was just curious as to whether there might be some good reason NOT to try using a tether.