generally, you don't need to consider the self-weight of the beam. The self-weight of the beam should be only a few kilograms and the load is uniformly distributed, while the external load is usually hundreds or thousands of kilograms. Compared with the external load, the self-weight of the beam can hardly be considered, which has little influence on the couple generated by the load, so the error of the experimental results is small.

if only the stress of the couple is calculated independently, of course, if the combined stress is to be calculated, the self-weight should be considered.