As someone also "in the biz" I do not expect, but obviously really appreciate tips. After several years, I've (or am in the process of) adjusting prices to reflect my own costs and what I feel is worth my time/knowledge. I've had tours that people seemed to love and received no tips. I've gotten tips that I felt were ridiculous. It just really varies. But I certainly do not feel that it is owed me. People come from different cultures and have different tipping schemes, so I make sure I'm paid what I need to be in the prices I charge.
That said, I work for myself, I guess you could call me a "freelancer." If you are taking a tour through a company and the tour guide works for the company, if they are competent I would definitely tip them. Unlike me, they do not get to set their own hours or prices, and the company obviously takes a cut. Even if it's just a euro or two, it's helpful and they will appreciate it.
And of course Jo is right that a good review (or two) on the various social medial sites is better than a tip, so if you are on a budget and really enjoyed your tour, please leave reviews because that gets us more business and is incredibly, incredibly helpful.