I don't do competitive dog training. I was interested in doing Rally with my Beardie but he isn’t suited for it, which is fine with me. I am very happy with simply having well-mannered family dogs. That is my focus with clients as well.
I don't demand or need perfection in the behaviour I ask of my dogs but it I do expect to get whatever behaviour I ask for.
If I did Rally with my dog, I would need to have certain behaviour under strict stimulus control.
What is it and why would I need it? Having behaviour under stimulus control means there are rules governing it.

Karen Pryor lists 4 rules, which govern true stimulus control. 

1) The behaviour always occurs immediately upon presentation of the conditioned stimulus. 
2) The behaviour never occurs in the absence of the stimulus. 
3) The behaviour never occurs in response to some other stimulus. 
4) No other behaviour occurs in response to this stimulus. 

If I was doing competitive training these rules make sense.  Take the Rally example: in Rally the dog needs to jump over a hurdle. Let’s see how the rules would apply.

1) The behaviour always occurs immediately upon presentation of the conditioned stimulus (the dog jumps over when you say “over”)
2) The behaviour never occurs in the absence of the stimulus. (The dog does not jump “over” anything if you don't ask for it)
3) The behaviour never occurs in response to some other stimulus (the dog never jumps “over” if you say down)
4) No other behaviour occurs in response to this stimulus (the dog doesn't lie down when you say “over”)

For many people, myself included, we have no desire or need for full stimulus control in daily life. 
Rule #2 especially is not realistic unless you add  " in a training situation" at the end of the sentence, especially when you think of an everyday behaviour such as “sit”.  It would mean your dog could never sit unless you’ve cued it. 

Many people I know, dog trainers included, do not have behaviours under full stimulus control nor do they care to. 

I think what most people are happy with is to have a dog who responds to a cue when given and don’t really care if the dog offers the behaviour outside of a training session. 

Dogs trained without force or fear of punishment, are quick to offer a down, roll over or high five in an effort to earn reinforcement. That’s not something I will ever discourage my dogs from doing.

I reinforce uncued behaviour on a regular basis because I want my dogs to know I like it. I may not use food every time but I will use praise or toss a toy or give a belly rub if that’s what they like. Having a dog who enjoys figuring out what works is awesome!
There are some dog trainers who may shudder at the thought of dogs not being under stimulus control. That’s okay. Everyone has different criteria for their dogs.
Right now I can’t think of a single behaviour I need under stimulus control. If circumstances change, I will too. 

For a different slant on the same subject of Stimulus Control, read Eileenanddogs Blog.