>i was told that (over working) chest and biceps can make it
>worse
I don't see what connection biceps would have to it, but definitely overworking chest and/or front shoulder contributes to it. And since most daily activities for most people involve more pushing and leaning forward and sitting than pulling (unless you're a fisherman hauling in nets all day, or a rower, or someone who saws down trees with an old-fashioned saw, perhaps) the chest and front delt are often too tight from that, while the rear shoulder and upper back are relatively weaker, which leads to bad posture as well as shoulder problems (usually some sort on impingement).
I think it's very important to keep the chest/back imbalance in mind, and avoid overdoing chest work vs. back (which usually means doing a bit more back than chest, and definitely NOT more chest than back). I also believe, along with Jerry Robinson and some other weight training authors, that isolated anterior shoulder work is unnecessary, because this muscle gets enough work when the chest is worked.
In answer to your question: I have had bad-ish posture in the past (especially times when I haven't done resistance training for a while), but not recently, and it's gotten even better since I've paid more attention to keeping scapular retraction during all upper-body exercises, and including upper back, posterior delt work in my workouts (and reducing the number of sets of chest exercises in some workouts, like Slow and Heavy, that do more chest than back.