If you are practicing on a block resting on the ground, it will be quite natural to lean. However if you are practicing on a target at breast height and are leaning, it will add unnecessary breaks in your form particularly at your bow shoulder, and straightening up your torso would then improve your effective shooting range by tightening your groups at longer yardage shots.
Also, if using a heavy bow, you do risk an injury to your front shoulder that would have been prevented otherwise.
I only lean forwards when shooting downwards, by bending at the waist which keeps my shoulder in the best position as if I were shooting across flat ground at breast height. I only lean backwards, similarly, when shooting up, and for the same reason. Likewise, I only twist my waist, when I am unable to move my feet, such as in a hunting situation when I am unable to do so because of crunchy leaves.
In short, it is always better to bend at the waist or twist, than it is to move your bow arm out of form.
Similarly, if you are leaning when it is not necessary, it is forcing your bow arm and shoulder out of form, and your accuracy, range, (and possibly even the life of your shoulder) would immediately improve after a weekend spent correcting the habit.
-Silent