I think most buff pilots use 50% unless they're flying a very short hop, 25% won't get you far even if it's an attack on a single airbase.
It will get you to 15k several sectors out to bomb an airfield and back. That's all you need. You only take more if you're climbing past 20K and going across the map.
One thing you need to remember with bombers is each selection of fuel will (in some cases) drastically change the climb rate of a bomber thus increasing the time needed to get to altitude. Forcing bomber pilots to take 100% fuel would mean instead of flying a 45 minute sortie at 15k I would end up flying a 1hr 30min sortie because of the extra time needed to climb. Heck going from 50% -> 75% in a B29 forces me to take off 2-3 sectors further back than had I taken 50% fuel.
That's the very POINT!
That's WHY you make them take 100%. They could never climb to 20k inside 20-30 minutes in real life. Why not strap RATOs to them? Simple: Because that's not how they worked. Taking less fuel is like a constant RATO in climb rate.
Forcing them to take 100% means they will either spend more time climbing (leaving them more vulnerable at auto speed before they level out) or they will level out SOONER to drop on their target in time. The end results are: It slows down the climb rate, and encourages bombers to level off sooner.
No more 35K Lancasters. No more 32K B17s floating so high you can't aim up at them let alone get up there to kill them (and if you do you get shot down 2.0K out because the air is so thin up there). No more climbing 1500fpm with a full bomb load in record time that would have made WW2 pilots drop their jaws. These are not Komets. These are not Ta152s. They are B-17s. They are B-24s. They are B-29s. They are Lancasters. They should at least superficially perform like their real-world counterparts.
Full fuel doesn't get us there, but it's a very large step in the right direction.