I'm not a big fan of thinning so I don't have much experience. The only problem I've had not thinning has been when 2 short stemmed apples stay on the same cluster, if codling moth doesn't lay for both fruits and a larve doesn't travel between fruits something like earwig will move into the cavity between stems and feast.
I'm not even sure if what I think is June drop is June drop and I'm not blaming curculio drops on June drop. Most of my "June drop" has been curculio bitten in the past before I started spraying but a lot of the ones that didn't drop have been bitten.
It looks like Once and Done is doing a good job against the curculio so they shouldn't be doing the thinning for me this year.

Assuming what I'm calling June drop is June drop the tree hasn't been too good at selecting. It leaves a lot of multiple apples per cluster some places and empties clusters other places.
I want to leave the tree I'm asking about heavy with fruit but one fruit per cluster.

Right now the tree is dropping what I assume are unpollinated fruits. The ones that are setting are gaining size quickly (about the size of a lima bean if lima beans were round).

I do thin heavily. If one leaves more fruit then the chances of some drop would likely go up. But maybe the tree would be trying to tell you something.

The Fruitnut

I had the same concern last year with my first apple crop. I sprayed with insecticide and then bagged the good looking ones with ziplock bags as soon as the apples were big enough to do so, and thinned out the remainder. My concern was that I would lose a lot of the bagged apples in June if I thinned to the ones in bags before then. Fortunately this did not happen. A few bagged apples came off, but they all had insect (curculio) damage I had not noticed. I thinned aggressively, following Jellyman's advice to leave one apple about every 6 inches. I did not get a lot of apples -- it was their first year -- but I ended up with some nice, defect free apples.

I think I definitely want more than one every 6" where I have the chance.
This is an espalier that doesn't like being pruned so small and I'm trying to stunt its desire to grow.
So as long as I can avoid the bug haven of multiple apples per cluster I want as many as possible.

It looks like I have it thinking about making spurs so as long as I time the pruning right I can start thinning branches without it sending all the spurs to branches again.
Hopefully between the two methods next year I won't have to try to get a lot of apples to stick and use a distance method of thinning.

I doubt if I would even get 1 bushel off the tree at 1 per cluster so I wouldn't be pushing its ability too much. When it had the desire to fruit instead of grow branches it would put out a bushel.