In reality, in most cases you would A rate the smallest power plant that gives you the power needed in the ship for heat efficiency reasons for any type of ship.So, the B rated modules are more desirable for combat ships and A rated would be for (as an example) for max jump range because of there light weight. I think I'm starting to understand the little quirks of this game (at least the outfitting).
You choose D rated life support and sensors, engineering the D rated sensors for longer range on a combat ship. This is better than light weight engineering on a heavy A rated sensor module.
On a ship you want best jump range D rated smaller thrusters. Same for power distributor - goes for any ship that doesn't need weapons or mining lasers.
If you want to protect modules in combat, shields, shield boosters and shield cell banks to keep shields up. If the shields do drop, putting 1 or 2 module reinforcement packages in the optional internal slots helps.
If you fit anything other than A or D rated core modules, it almost always because your cutting costs.
It's more varied for optional internal modules. Example: B rated collection limpet controllers give limpets longer range than A rated, but A rated have longer life. When mining, I find B rated for the range is better because limpets are cheap and you can carry plenty. Doesn't matter if their life is a bit shorter.