I'm actually working as a guide doing free walking tours. I was curious to see what kind of response people had first though.
Half the posts in here don't seem to know what it actually is. for the most part though, it's a walking tour, but your free to pay what you think it's worth. (Bad guides make shit money).
I started with a company called Sandemans. It's true they don't get paid. They actually take about 3 euros per person from the guides tour as a marketing fee. So if you have ever walked away without tipping, then you just cost someone 3 euros. How do they know how many were on their tour?? You signed up and were probably in their group photo(which people like to do as well)
I work with a smaller organization now in which I'm part of the management team, so our marketing fee is smaller and goes to something I support more than mark Sandemans pockets.
On my tours though, 5 euros is usually a disappointing tip. Although I live in a more expensive city, where wages are higher and cost of living. So maybe it depends where your touring.
I just wish the conversation was more out spoken, at the moment it's hard to find a good resource on what you should tip these guides.