We paid 200birr for fixed price yellow taxi at the Addis airport this week, based on a printed list from an "official" guy standing outside. Our hotel was well known and in Bole. On the return to the airport a few days later the fare was 150birr, called by our hotel.
I am not sure from your original question if you are planning on taking the bus to Bahir Dar, but if you plan to fly in, we had an issue getting a taxi at the airport there for a fair price - they had us over a barrel. I strongly suggest you pre-book accommodations in that city to get the airport pick-up, which seems standard, even for mid-range hotels. The Selam bus office in Bahir Dar is in the middle of town, by the market and close to the bus station. I did not see the Sky office. In Gonder, both the minibus and the larger bus companies you mention drop off right in the center of town. We took a minibus. It was overpriced.