Who still pre-cuts pickets? I tried that once and ended up with a wavy fence that looked drunk. Now I always trim as I go, and it saves so much time and hassle.
Honestly that "drunk fence" is the perfect example. I see this everywhere where people try to plan every little detail upfront instead of just adjusting as they go. Tbh it usually just makes things harder.
Watch a newbie try to pre-cut everything and you'll see the problem. They measure once, cut fifty boards, and then the ground dips or the post is off by half an inch. Suddenly every picket is either floating above the rail or hanging too low. Trimming as you hang them lets you follow the actual line of your yard, not the perfect line in your head. It's the difference between a fence that looks built and one that looks thrown together.