Thank you William, for saving me the trouble...
My one fully engineered ship - used for PvP regulaly - has one extra h/s ammo & I fire two 5A SCBs using one h/s....trying to fire two without a h/s would be catastrophic & I can't spare another utility. Hence 'A rated' SCBs are the optimal choice for me.
<Amended Morgan Freeman meme: 'He's right in certain circumstances, you know!'>
The weight penalty with all B modules can be a significant factor too, especially since RNGineers, when you get a poor roll (as is usually the case) on the thruster optimal mass stat from grade 5 DD upgrades.