I tend to sit far back on the bench too. However, I don't slouch at all. My back is in it's natural shape which it should be when you play piano. I feel like sitting too close would make one slouch more than usual while if it's too far back you would tend to lean forward. The keys are too close so you slouch in order to move back your shoulders.
I've been told that you should sit on 1/3-1/4 of the bench when you play piano. Any more than that and your feet won't be providing enough support when you play.
Instead of how close you sit, you should experiment with bench height if you can. Having a bench that is too high will put weight on your arms and hands which might be uncomfortable to play with. On the other hand if it's too low, you will begin to raise your shoulders which will tense you up.
Find the right bench height first, then sit at a distance where you are well supported by your feet and have a straight back. But keep in mind they are directly related to each other. I find for unadjustable benches that are too high I have to move further back in order to maintain a straight back and comfortability at the piano. Of course there's only so much "wiggle room" for adjusting to high benches.
Which brings me to a rant.
WHY THE HELL DO THEY MAKE UNADJUSTABLE BENCHES WAY TOO HIGH? Like seriously some of these benches only a little kid could play comfortably on. I've had benches so high that my thighs have been squished between the piano and the bench. I mean if it were lower, you could just put books on it or something to make it higher but NOOO piano bench makers have to make them ridiculously high for everyone. And I'm not even that tall, only 5'6" and these benches are still way too high (oh yeah, the taller you are, the lower your bench has to be).
Okay rant done.