Clean mapping guide.
Very nice tutorial, lpfreaky. This is almost the exact method I use, except that I usually also texture the sides of the initial block with either anim_wp/framework/squarebeams or (depending on the theme) anim_wp/framework/squarebeams_rusty_01. That way, if I later decide to delete some blocks to make a hole (because of decaying or Wheatly damage), the adjacent ones will already have a nice looking edge. I usually also texture the back with one of the backpanel textures, in case the outside of the room becomes visible for any reason. (Cave-in, seen through a hole in the next room, etc.)
For optimisation, you can always merge blocks and apply nodraw when the map is close to finished. This is usually less work than doing that from the start and having to add back detail later on.
I'd suggest you to create own topic, I wouldn't be so happy if someone else is posting tutorials to my topic.
100% agreed. Making your own topic makes it a lot easier for people to find your tutorial as opposed to it being buried in another thread!
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
You should do this sparingly, its good to get ideas, but noone wants to see exact copies.
As long as you're not ctrl+c, ctrl+ving you should be fine. BTS chambers tend to be very unique in terms of different ways you can detail them. I'm going to try making a BTS tutorial and post it here.
I don't mean here, I mean in the forums.
Ok, excellent 
Just making sure.
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
You should do this sparingly, its good to get ideas, but noone wants to see exact copies.
I wasn't implying that, and you are correct, never copy... because we can always spot it
ETA?
PortalStories.com
"Oh, in case you got covered in that repulsion gel, here's some advice the lab boys gave me: DO NOT get covered in the repulsion gel."
When will this be updated? It probably wont though, because of the new map editor, but I was hoping for an update.
I don't know if I got time this weekend, but I was planning to do it sometime soon. (within the next two weeks!)
You are mostly right: you're using dev textures; this is common with other source games but since the blocky nature of most portal 2 textures another texture is also just as fine.
Your strategy for creating walls, floors and ceilings is most likely more efficient. And when I started this guide I knew I'd get at least one reaction like this. The main advantage of this method though is that people start to think in bigger blocks ad that they easily see that their textures are misaligned. A final advantage is that by using 128x128 textures It's easier to actually make all your textures fitting. The main point of my strategy is to get people more aware of how blocks can influence the final texturing and that a 128x128 block (either a separate brush or part of a bigger brush) is often the best choice.
About the dark/light metal thing: After seeing your pictures I guess I have to agree that light metal indeed looks better. I will change this!
If the player can't see misaligned textures, maybe they should get glasses. They're glaringly obvious not only in the editor but in the game too. Another advantage to my method is that the player isn't restricted by 128x128 blocks, they can also use 32 unit blocks to add in small panels. Newbie mappers should start out with the proper techniques that way when they become more experienced they already know these things and won't have to switch over. You don't train a dog by tying string to their hands and animating them like puppets, contrawise you don't train a mapper by making mapping moron proof.
Always aim for minimal brushes, it makes your maps streamlined and more compatible. A solution to a problems isn't always the easiest one. Mapping has always been a long process and in order to produce quality maps you must fuss over every detail to make your maps perfect. If a test is prone to glitching, fix it or scrap it. That's what separates the good maps from the great maps.
-GLaDOS
Lpfreaky90's method will help me a lot! The way you map kind of reminds me of the workshop puzzle creator, which is easy, so I like this method. 
Oh, and in case you're interested, I do know what brush is, despite my ranking.