A simply supported beam of length l carries a uniformly distributed load
Answers
Case 1) Assuming the section is made of reinforced concrete,
Shear is more closer to supports, but that does not mean that is where shear stress problems can occur !! (for a smart engineer, that is)
Ideally, shear stress problems must occur all along the length of the member (except in the mid-region where the shear is small enough for the section not to have shear reinforcement / minimum shear reinforcement). This dictates the provision and curtailment of shear reinforcement along the length of the section.
In practice, it is up to the engineer whether to make shear checks every 1 m / 2 m etc. and curtail reinforcements to save cost.
Case 2) Assuming the section is made of steel (hot rolled - from catalogues)
It is a common design assumption that “flanges take flexure” and “web takes shear”. My limited experience tells me that it is relatively rare for web sections to change along the length, even for long members like bridges (changes in flange is common !). So I would say a simple shear check at the supports should suffice.
If the section is class 4, everything gets f**ked up. Local buckling starts governing and effective web heights have to be calculated. Note that these buckling checks are performed for compressive stresses in the web which are a direct result of shear stresses !
Construction methodology matters. If the beam is cast / assembled and then rolled on in place (like incremental launching), critical shear stress can occur anywhere along the whole length and checks have to be made for each stage during the launching process ! (just for self weight, through)
If this were an interview / classroom question though, the answer “close to supports” should be enough :) But I wanted to share a glimpse of the big picture.