Although I see the value of a private guide, that would be hugely expensive for me as a solo traveler. My preference is for walking tours organized by the local tourist office if I'm looking for something general. I have a lot more confidence in the information those folks impart, and my observation is that the size of the tour groups is more likely to be reasonable than is the case with "free" tours. In Barcelona there was a significant difference in the group size.
The tourist office tours usually cost around 15 euros. I probably tip about half the time--maybe about 2 euros, depending on whether there's a decent opportunity to do so and what the other travelers do. Given the (comparative lack of) tipping culture in Europe, I don't think tips are necessarily expected on group walking tours like that when you've already paid for the tour.
I have an objection to the commercial "free" tour model, which forces guides to beg for tips; not only does it take advantage of the guides, the participants are virtually always subjected to a lengthy pre-tour explanation of the philosophy behind the concept, which cuts into the actual tour time. (Those commercially-organized "free" tours are not the same as the occasional tour offered at no charge by a local tourist office or other non-commercial organization as a means to encourage tourism.)
Rick has explained that guides on those commercial "free" tours start out each tour significantly in the hole, because they are charged 3 or 3.50 euros per participant by their company. Anyone tipping the guide on such a tour 5 euros is effectively paying 1.50 - 2 euros for 90 minutes (or more) of the guide's time. I consider that seriously inadequate; I hope people do that because they assume all the money is going to the guide.
I will take "free" tours if that seems to be the only option in the city--which was apparently the case in 2018 in Wroclaw. In Wroclaw I took three of the tours, including Bridges and Islands as I recall. I tipped at least 10 euros for each one, maybe 15. I did enjoy the Wroclaw tours, though the Dwarves and Communism tour was heavily tilted toward the dwarves. Still fun, but I would have liked more on the Communist era.
I appreciate being able to connect with other visitors and with the guide (remember, I travel solo), and I like the opportunity to ask questions. Guides often share useful information about their favorite restaurants and cafes in passing, no matter what the topic of the tour. I love that, because I'm just not willing to pay the prices usually charged for food-oriented tours.
Theresa, you've mentioned Lisbon and Wroclaw. What other places are you going? Folks might be able to make suggestions for those cities. Certainly London Walks (15 GBP per person) are excellent, with dozens and dozens of topics to choose from.