It's always important to be aware of how you're blocking around holes. The sandpaper always wants to cut the edges of it too low. Depending on the situation, you might want to sand up to the hole, rather than over it. A lock cylinder hole is another example. Be cautious around anything like that so you don't end up with a slight hallow spot around it. Also, it doesn't matter how big of a block you're using, the sandpaper still wants to follow the hole.