Weight or the lack there-of is crucial for a combat ship, or any ship for that matter, it helps with keeping the ship lighter and hence keeps the ship agile which is important for combat and avoiding interdictions, and increases jump range which in my T9 can mean the difference between 3 jumps or 2 jumps to return home, One jump less is preferable with stacked missions which have interdictions.
Really, you're dead certain of that.
If I'm moving about locally in my T9 I always refuel by 10% for the one way trip (keep the weight down).
If I'm doing local combat, which I do frequently, I always keep the weight down by refueling by 10%, (to keep the weight down).
It already is but you haven't noticed, It's on the main mission board, along with "Reapair All" and "Restock", that way those who don't wish to optimise don't have to delve into Advanced Maintenance, so you're well catered for.
I like the option of having a big Tank for long distance travel and exploring which is when I'll Refuel All which I can do from the Main mission board. Just because I have a BIG TANK doesn't mean I have to fill it.
EDIT: a brief description on how a commercial Captain decides whether to take additional fuel over and above computer generated flight planned fuel.
Taxi fuel, busy airport add fuel for traffic jam.
If departure/climb weather is bad add fuel for weather avoidance.
If there's bad weather enroute add fuel for weather avoidance.
If destination weather is bad increase fuel for possible Hold.
If destination alternate weather is bad add fuel for Hold.
Here's the catch, half the fuel will be burnt to carry said fuel, so if I want 10 tons extra at destination I have to add 50% because it is going to cost me that to carry the fuel.
And.. many times the salty, experienced Captain will look at the flight plan and say "add 5 tons for my grandmother".

) which means I know this plane is bent and uses more fuel than indicated in the correction factor of the flight plan.