Basically, taking a taxi is not an option, unless under specific circumstances, so therefore, very rarely do I take taxis.
When I take a taxi in Germany, I never speak English with the driver, always German, whereas, when in France, the Mrs does all the talking with the driver only in French. Doing that makes a difference.
Traveling solo in Germany before I get in, I always ask, how much would it cost, ie from here to "B" "so ungefähr?" If the price quoted is reasonable, worth it, I get in. Male or female driver I usually engage him/her in a bit of conversation, depending, and I watch if the meter is set.
The last time I took a taxi was in Vienna in 2017 as the train was more than one hour late in arrival at Wien Hbf. Since I knew pretty much what the fare was from Wien Hbf to the hotel close to Karlsplatz, I said nothing when the girl didn't turn on the meter....bad mistake.
If she had intended to rip me off (why wasn't the meter turned on?), she may changed her mind when I mentioned I was from here to her surprise because she wasn't used to foreigners talking in German to her. This girl herself was not ethnically Austrian, which I had guessed all along. When she reached the hotel, the price she gave me was less than the price I had in mind, then she says, no change. Definitely, I was not going to pay her with a credit card.
No, I don't believe I've ever been ripped off taking a taxi in Germany, what they tell you in terms of the fare, be it DM or Euro, I have found the estimate pretty dead accurate. If it's quoted at 12 Euro, the ride ends up at 11-12 Euro.
In France and Paris too, mostly good experiences, but a couple of drivers were not too easy to deal with, had an attitude, etc. I let the Mrs handle all these situations in France as she is fluent in the language, Those times in the past I had to take a taxi in France when solo, I tried French and relied on the driver's English...it was fine, say from the harbor in Boulogne to its train station.