Hello! We just got back about two weeks ago and taking an occasional taxi was one of the best things we did! Melt downs were avoided severl times, esp. late at night.
We took one from DD-they were lined up in the main parking lot on the opposite side of the street from the buses.
We also took one from MK. We went by boat over to the TTC. Then walked through the main gates toward the parking lot and there were taxis all lined up. The fares were about $9 for us-definitely worth it to save sanity. What I would do would be to ask BEFORE you go into the other parks just where and how to get a taxi. Then you will know where they are should you choose to take one.
It depends on which park. The taxis are right outside the park at MGM, DTD (several places) and one can always be called by a valet at resorts.
At MK (if I recal) they weren't allowed to drop you off where the buses are, so I think you have to get to TTC to snag one. Although I'd just head over to one of the resorts (CR) and get a taxi from there.
I can't remember about Epcot (I haven't been in through the front entrance in years!).
Taxis are great and you never end up spending more than $10 or so.