I was just wondering what everyone's views are on documentation relating to a web project you are/have been working on.

It's occurred to me more and more the importance when working on a project to keep a note of everything you are doing, and the steps involved. Well, I don't literally mean everything, but whether it is instructions, diagrams or storyboard designs, they are all part of the pre-planning (or post-planning) process which help you or others understand the stages involved.

One of the habits I have (and I'm sure others do) is writing the supporting documentation after completing a task. I know this is not good practice, as you are bound to miss out on key elements, but it's just so tempting to do.

I've actually been writing out the steps formally in Microsoft Word I have taken to make a menu in Menu Builder look and function as I desire. I don't mean just in the software, but the CSS changes I've made and justification for those changes. I shall be doing a similar thing with my website created in RSD after. I know I should have done this during the process of development, but that's something I will just have to keep working on.

So is documentation important? Or do you just prefer to crack on and develop a website with none of the planning hassle.

I have files (separate from my websites) that I keep bits of (commented) code in that I might want to reuse. I also make notes as needed in notepad and save those text files with my code files.
Nothing more aggravating than trying to duplicate something on another site and realizing you don't remember how you got it to work the first time around
I recently started a "sample site" to experiment on so I'll have working code examples with the content being notes about the code. Someday I might get going with Github or JS Fiddle, but for now I prefer to keep my 'frankencode' to myself.

Whatever works for you/makes things easier is a good thing to get in the habit of doing.

Documentation IS important, no doubt about that. I usually have my files littered with comments in order to document what I'm doing. Sometimes I write the comments separately and print them out, especially if I have to continue work on a site that I have created a while before.
I guess most people have their own system for this, and what is right for one person might not be right for another.