A lot of great advice has already been given, but I'll chip in too
Coal is driven almost 100% of the time without blinkers, but according to his previous owner if he has blinkers on he will throw himself into the cart. Apparently he was an amish pulling pony that was overburdened so he was throwing himself into the harness. So the previous owner re-schooled him without the blinkers. He's the coolest in temperament driver I've ever had and so it wasn't a big deal to keep the blinkers off. Especially when I had no intention of showing.
With the other horses/mini's it tends to vary on the horse. All but one mini was driving in full gear (blinkers and all) within a week of being introduced to driving/broke to drive. Whereas one mini took several weeks before she "calmed" down in the cart with the blinkers on. She was a very overstimulated horse with an abused history that the raise of your voice could send her into a frenzy. So it was helpful to delay the blinkers until she was comfortable and aforementioned, she had to mildly retrained once they were introduced. She still has her moments of panic, but is much easier to bring back down to earth and calm her down.
Willow, my cob, is ground driven in blinkers and full gear. Since her incident she hasn't been hooked to the cart, but my plans are to drive her without them until everything is underwhelming to her. Frankly, if she had had the blinkers on during the incident, it probably would have been incredibly worse not being able to see what was going on.
I think the horses are pretty good about letting us know when they are ready for step ups and if a step down is required
I have been absolutely impressed with how mini's handling being broke to drive and how quickly they take to it. The bit seems to be the biggest obstacle for most of them.