Yes, Zero is the target. You can, of course, experiment with that depending on conditions ora particular "look" you want but yeah, zero is the goal.
A slower shutter speed will allow the camera sensor to receive more light by "holding" it open longer.
The rule of filming at a shutter speed of 2X the frame rate is very hard to achieve in a bright condition because if you slow the shutter down that much...it will wash out the exposure/brightness UNLESS you can control the aperture! THAT is where the ND filters come in. They will block the light so you can achieve that shutter speed.
All this is valid for filming.
Still images, however, require a faster shutter speed because of the "moving of the drone" (it is not on a tripod). A slower shutter speed will create a "smoothing" for example..like in a waterfall when you want that smooth look ...but if the drone is moving, even slightly in the wind,etc it will cause a less than sharp image with that slower speed. Again, a filter will allow that slower speed by not letting in all the light but then the capture will be susceptible to the motion of the drone.