I used to help my Dad trim trees when I was a kid. He was very particular about it. He carefully sealed each cut that was more than about a inch in diameter. He also sterilized his saw after each cut because he didn't want to spread any tree diseases.
So I've always thought that you had to seal trees when you pruned them. But I have a neighbor that insists it's not necessary to seal them. So I may be guilty of "old school of thought." But I know there are some serious tree experts on this forum, so I thought I'd ask. I'd rather not spend the extra time and money on tree sealer, but I also want to do what's best for the trees.