3 Questions about this Unwrapping technique

I still am very unsure about some things. When it comes to unwrapping I always think I’m doing something wrong.

1. Is mirroring UVs a common thing?

I never used Blenders baking options before because I think Substance Painter does a better job and it’s much easier to bake there. But I noticed that I don’t get a clean baking result when I lay some UVs on top of each other. I also never saw people laying UVs on top of each other in other videos. So should you only mirror UVs when you use Blenders baking tools or is this a common thing in the industry?

2. Why not simply use “Unwrap” and “Average Islands Scale”? 

In the past I always marked necessary edges as sharp/ Seam and simply pressed Unwrap. Then I adjusted the “Margin” and sometimes used the “Average Islands Scale” option which seems to scale all UV Islands accordingly so that one square of the color grid has the same size on all objects. The result always looked pretty decent to me so I’m wondering what’s better about moving and adjusting the UV Islands manually?

3. Do you really need to mark all sharp edges as Seam? 

Also a thing I’ve never done before. I learned for example that when you unwrap a simple cube, you don’t mark every edge as Seam even though every edge is sharp. Sometimes I just use 1 or 2 Seams for a larger area with multiple sharp edges. Example: 

I hope it’s clear what I mean and thanks for the course.