@Clemens,
In order to be succesful as a freelance graphic designer you have to get paid for the work you do. So, I always balance the needs of the client within the scope of their budget. By default I only take on work that can afford a presentation that includes a full color version of the logo, a flat color version, and a black and white version.
There really is no limit to how many things you can put a logo on or what will showcase the logo. Placing it within the clients budget makes it more realistic as to what they will pay to see.
A second consideration is to look at the type of company and if they want to work into the budget seeing the logo on merchandising, or on a website, or if you should design business cards.
In this 2 part logo design tutorial you can see what I presented to a client. This was a small project. A bigger budget customer might have wanted to see more.
http://aiburn.com/article/bzzz_creat..._design_part_1
http://aiburn.com/article/bzzz_creat..._design_part_2
Hope this helps. Thx.