I remove everything so that I have a very smooth and absolutely clean surface to put the tape on. It also provides a hard/smooth surface to use the pressure roller to make the tape stick better. If the stuff under the tape is soft, you really can't put a whole lot of pressure on the tape to make it seal.
Takes more time and effort, but it's better to do the job correctly once, rather than have to do it over again because you took short cuts.
ron