I'm actually a huge fan of "overtraining," which is training the dog to a level much higher than that needed for daily life or whatever . For example, if you need the dog to be able to hold a few quick sit-stays or down-stays, then your first thought might be to teach an up to 30-second stay, and then add a few basic distractions such as a food drop or a tossed toy. Once you've gotten your dog to do this 30-second stay with a couple of distractions, you figure you're good to go.
I'd suggest that if the 30-second stay with two distractions is the very best that your dog can do -- it may not actually hold up in the real world. That 30 seconds represents a "stretch" for your dog. What if the clock ticks over to 31 seconds, or what if a new distractor that you didn't train for appears?
I most frequently see the benefits of overtraining in the Rally Obedience ring. Many people like Rally because it's less formal and not required to be super-precise. So you won't actually get points off if your "heel" is not very fancy or sharp or if the dog doesn't maintain perfect eye contact or his left turns are not done as pivots. But when you add the stress of the ring, your dogs "best" behavior can break down alarmingly easily. Even uncontrolled things, like the loudspeaker crackling on or a burst of applause from the next ring. If your dog who was trained precision heeling falters and momentarily lapses into "good enough" heeling, you're ok with no harm done (other than you probably want to go back and do some confidence exercises with sudden noises so he's not scared of loudspeakers/applause in the future.) If your "good enough" dog falters though, you will lose. So keeping in mind the likely possibility that your dog will falter at some point is really handy.
For pet dogs, the main benefit of overtraining is simply that your everyday life will be soooooo much easier. You won't have to worry about "What if he doesn't respond? What if he can't do it?" You know he can do it, because he's done way harder in the past.
So, feel free to train your dog to "Leave it" as hamburgers and cheese cubes are dropped from above or literally thrown at him, come when called past or through dogs galloping off-leash, and "Let's go" (loose leash walking) while you creep along at a snail's pace. Overtrain and be delighted!