TECHnically, we genuflect toward the tabernacle whenever we enter or leave any pew. We should also genuflect before the tabernacle whenever we cross the church right in front of it, and the same thing goes for bowing to the altar.
However, the custom of the church in the US is usually only to genuflect at some times and not at others. Some parishes still genuflect while leaving for Communion, for example, and most people end up genuflecting after Communion because they’re kneeling in the pew anyway. Other parishes have been sternly instructed not to do this. At my old parish, lectors always genuflected when getting up to do readings and getting back into the pew, and then bowed before the altar when going up to the lectern and coming back. At my current parish, we lectors have been instructed not to genuflect. Out of obedience, then, I don’t.
But you should still always genuflect when crossing in front of the tabernacle when the King is within; and always bow in front of the altar. Always, always, always. If anybody tells you different, you just do it anyway. It’s not optional.
Re: bowing to the altar, it’s not because the altar is “the focus”, and it’s not only during Mass. A church altar has been made intrinsically a very holy place – the place where Heaven touches Earth, where Christ comes to us with Real Presence, and the sacrificial actions of the Mass are performed by Christ the High Priest. As long as that altar hasn’t been deconsecrated and you’re walking in front of it, you’d better be bowing your little back. Profoundly, if you can do it; at least nodding and holding the pose if you physically can’t.
And I’ve come to the conclusion that you may as well take the time to do a good genuflection or bow. It’s not like bobbing your head or knee is so much faster than doing it right.