How to connect USB led strip via battery
If you are going to use this continually, I suggest to use a buck converter along your 9v battery. Buck converters will reduce your input voltage to a level that is interesting to you. In this case, 5V. 
The other thing that makes me think that this is a interesting option is the fact that the potentiometer used to variate between voltages on the buck converter is prepared to this application, in a way that it will have small currents flowing on its terminals.
Using a potentiometer in series of your voltage source will not be a good idea, since all the system current will be flowing on it and the current for the normal potentiometers is low.