That's a good question, in general, designing it as R-beam with single reinforcement (only tension side counts) is the safest and the most conservative way to go (so it is my first choice all the time), if you are limited with the amount of steel and the beam dimensions (and you need higher strength) then you may need to consider adding that extra strength given to the section by the use of compression reinforcement (doubly reinforced) and by using the T-beam analysis. I hope this explains it...