For me it depends on the flavour. For instance if I were using menthol I would probably use a new tank. But I don't usually mind using the same tank for a fruit when I had been vaping a dessert before, or vice versa. Sometimes the combination is quite delicious. That said, unless the initial juice is very overpowering, the flavour does get replaced by the next one pretty quickly. One other trick I use is to wash and dry the various parts of the tank apart from the coil. The coil I wipe down very well and closing off the bottom i blow hard into the top end. That usually makes a lot of juice in the filling cotton or wick come out of the side air holes leaving the inside of the coil head relatively dry. Of course if you are using top coils like a the iClear 30 or something similar you can always wash and dry the wicks.