Hi, nick65go. Coding style is a big topic. Two main elements are formatting (indent style and spaces vs. tabs) and readability.
There are many indent styles. K&R and Allman are the two most popular. I personally prefer Allman. See here for examples of 7 indent styles:
http://www.terminally-incoherent.com/blog/2009/04/10/the-only-correct-indent-style/Spaces vs. tabs is a big flame war. I personally prefer tabs set to 4 spaces, but some people think that tabs are an abomination.
As for readability, code is written once but--depending on the code, licensing, and how many users/contributors the project has--the code can be read and edited thousands of times. Books could be (and have been) written about readability. I think it's safe to say that it encompasses knowing when to use comments (i.e., don't comment on things that are obvious but do comment on anything that another person--or you 6 months after writing the code--might be confused about) and writing code that's sufficiently explicit without being too verbose. Writing readable code is an art and it seems that one is never quite done learning how to do it better.
I think your question was specifically about formatting. This is true both for TCL as well as any other FOSS project: If you intend to submit a patch for a file, it is best to use the formatting (indent style and spaces vs. tabs) that's being used in that file. Even if there are no spaces where you'd expect to see some, stick with the formatting used in the file. If the file is inconsistent (tsk, tsk!), then I guess I'd use the formatting that's most prevalent in the file. As for formatting your own projects, the most important thing is to be consistent.
I hope that's helpful or at least interesting