Actually lowering to go forward is more complicated, for a very good reason. And I don't mean which way the knee bends, but rather the footwork. If you had your entire static bodyweight in your toe, and then lowered to your heel, you would actually have to move backwards most of a foot length. Going backwards this is fine, but going forwards? Do you want your weight to arrive and then move backwards a bit as you lower? No - which means that going forwards you can't let your entire weight arrive before you start lowering. That doesn't mean you lower while you still have weight on your other foot, but it does mean that you lower while your weight is partially unsupported - if you were already stable over the new foot, your weight would be in the toe and you would have to send it backwards to ever get it in your heel.
When you have your feet closed while up, you probably do have your weight completely in the toe of the arriving foot. But since the body isn't really moving much there - it's nearly stopped, the backwards movement of your weight as you lower isn't as much of a problem as when you lower from having your feet apart, say while stepping out of a chasse.
Also, some teachers will insist that you arrive and then lower. This is not the ultimate goal in timing, however it does help build your foot strength if you dance that way for a while. It just comes at the cost of having to break some of those habits to retime your lowering before you can dance well.