I am not going to pretend like I am in-tune enough with the physics of light to draw up lighting diagrams BEFORE a shoot. However, with almost every single image I create I draw a lighting diagram, usually after the fact. It's one of the most important things you can do as a photographer who works with off camera flash.
The process of lighting a shoot is fluid for me. It generally begins with a concept and a rough idea of how it will be lit. This is almost always just a jumping off point. The quality of light may not be how I imagined it, or there might be other factors influencing the light in a way that I didn't think of. So I adapt to the result that I am getting and make adjustments until I get what I want.
Once I have an image that works with what my initial vision was, I take a few minutes to sketch a light diagram.
I consider this a very important part of my process. It helps me analyze what worked, and frankly helps me remember how a shoot progressed. I often include notes about how the shoot went, where my light power levels were, what the sun position was (even if it didn't figure into my image). I also sometimes include stuff that should be included in the EXIF data, such as lens, camera body, etc. I do this because sometimes meta data gets stripped, or other bad things can happen and it takes only a second to write it down.
I think most importantly I do it because I like having a visual record of my work (aside from the actual images) that has some technical behind the scenes information, and also I find it therapeutic to write them in a notebook. Like with a pen. Low-tech to be sure, but oh so satisfying.
This is a practice that I recommend to any photographers, amateur or otherwise, who aren't in the habit of doing it. My notebook of choice is the Field Notes Brand 48 page Graph Paper Memo Book. I don't try to hide that I am obsessed with this brand. Mainly because they're wonderful, but also because they are small, so they fit in any bag. They are durable, and I adore the design sensibility and philosophy behind the brand.
So do you make lighting diagrams? If you don't, I think you should. It will help your creativity, critical thinking and possibly be a source of inspiration for future work.
Field Notes Brand - The best memo books around
Strobox - A really interesting online diagram/photo sharing tool for photographers
Also read This Article on taking notes