I was inspired by Mangetout’s post in this thread. Good question. I’ve seen plenty of trays of Napoleons and they always look perfect. The pastry is cut squarely with no cracks, and the cream filling isn’t oozing tasty goodness out all over the place. 
They’re a pain in the ass to eat 
, but apparently bakers know something we mortals do not. How do they do it? 
In a restaurant with individual servings, you would precut the puff pastry, into individual pieces before or after you actually baked them. If you have a long Napoleon, you would use a serrated knife and cut in a gentle sawing motion. If the puff pastry is fully cooked, you should get very little cracking.
No, you cannot cut them when they are cooked; the filling oozes out. You must make individual servings, otherwise I cannot imagine how it’s done. The pastry part is tough and the filling is soft. Can’t be done!!!
At one of the bakeries I used to work for, we cut the puff pastry into individual portions before baking. Afterward, we would slice each piece into three sheets, squeeze the filling on the bottom two pieces with a pastry bag, put on the top sheet, then ice them. We used warm ganache for the chocolate “feather” pattern, and made the actual feathering with a toothpick.