G'day Mike. I definitely compose the shot first, lock the camera/tripod in position and manually focused, then slide the filter down. I don't use LiveView very often, usually do it by eye. But last time I did use LV and it did seem to help with where to position the filter graduation on the horizon. That seems to be the trickiest part, but I think you're better off erring on it being to high than too low. Just because I don't like seeing that dark band on the horizon, makes the use of the filter obvious. Guess you could dodge it out or something, but better to get it right first.
Don't ask me about mountainous horizons. I've only used them on seascape/level horizons mostly so far, just getting used to that. That's why I've kept it simple.
Definitely be using the soft grad on horizons that are uneven I think.