It is interesting to see the variation between colonies and their use of propolis. I use a strip of hardware cloth to block the entrance during the summer (leaving a one inch opening at one side). It allows plenty of air to circulate in the extreme heat in this part of Texas. I have seen some bees use propolis aggressively to seal all cracks, modify the shape and size of the entrance opening, even make mini ducts to control air flow coming in one side of the entrance and out the other.
I have read that hives with more propolis are, on the main, more healthy than those using less. When you have a hive really making a lot of propolis, it seems to me they are doing very well. In the wild, bees often completely coat the inside of their cavity with propolis.
At 100* F, excessive propolis isn't a problem, but let it cool down to 60* F and the hive components really stick together. I looked in a hive today with a lot of propolis on the inner cover. I probably won't open it again until spring temps of 70* F or above, unless it feels light in early spring.
Last winter, I put a ball of propolis on top of the frames in a small hive. Bees showed interest in it all winter. When they had pollen substitute, they paid just as much attention to the propolis as they did the pollen sub.