It depends. I rely on past experience with the camera+lens+ballhead combination. For example, the 15-30 is a heavy lens with no tripod foot. It has never caused my larger ballhead to droop. With my travel tripod ballhead, though, the lens can slowly sag downwards, so I tend to shift the camera backwards to balance the weight.
My most extreme off-balance situation was for the August 2017 solar eclipse. I was using a lightweight tracking mount to follow the sun at 500mm. This was a horribly unbalanced setup, and I worried about the camera falling to the ground, but careful trial runs at home showed that it was stable enough.