I dropped paper towel behind the drum in the top load washing machine. Do I need to worry?

Maybe not.
Unplug the power from the wall.
Take a putty knife blade and bright light, find the catches at the front of the machine at the gap just between the top and bottom an inch or so in from both sides. Slip the putty knife in at the catch and apply pressure to the highest part of the catch while gently lifting the lid on the side until it releases. Do the same to the other front catch until it releases. Now hang on to the hinged door lid while opening the whole lid towards the rear and fish out the lost item. Gently close the whole lid and while it is resting on the catches, give the lid a final press down to latch the lid closed on the latches. Plug the machine back in and feel the accomplishment of a DYI'er!

I would rather you recover the errant object than risk harm from it getting into the motor. If it is too far to reach, use a yard stick with masking tape attached to snare the paper towel with the sticky side of the tape extending off the end of the stick.

Good luck!