I am not sure what extent this applies to humans, but I have watched hundreds of animals die, and have seen thousands more dead, and it is extremely rare I ever see something produce any waste post-death. I'd assume if your bowels and/or bladder is very full, it may release as the muscles relax, especially the bladder, but doubtful in most circumstances.
It may be of reassurance to fast 24-48/hrs prior to CTB, as well as keeping hydration intake minimal.