This is so basic, yet so powerful, that it's one of the few pieces of advice to appear in every pickup artist writing, video, etc. It makes you look overly eager to accomodate, too happy to get into her space, and submissive to a complete stranger. Also, how do you even know she welcomes it?
Even -- especially -- if she is a lot shorter than you, do not give into the urge to bend over out of politeness. If she likes you, she will move her head into your space. Tonight at '80s night, a peppy 5'1 teenager asked me wasn't I hot in my suit. I yelled my answer, but it was on the loud dancefloor, so she didn't hear (deeper voices don't carry as well). Rather than bend over, I stood how I was, and she grasped my arm, stood on her tippie-toes and placed her head near my mouth so she could hear. She did the same when she wanted to make sure I heard her.
I asked another girl in the group, who was 5'3, if her friend always asks so many questions. Again, she didn't hear at first, but she did the same thing: nestled close to my side, stood a bit taller, and placed her head just above my shoulder to hear better. There is an important aspect of social psychology at work here, which I'll cover in a future post.
If I were 6'8 instead of 5'8, then I would slouch over or something, since the average girl could not stand that tall, but for the vast majority of girls, you won't have to worry about them standing up to get near your mouth. I emphasize that you should always doubt your urge to be polite -- girls don't want you to do the polite thing in these contexts. It gives a girl an incredible rush to arch herself on her toes, grab hold of your arm for support, and maneuver her head close to yours, as though she were going to whisper a secret in your ear. Have you ever seen a slow-dancing couple where the guy's head was on her shoulder? -- No? That's why.
Girls love doing things that make them feel feminine, and having to nearly scale your body to hear you is one of those things. It's like how little kids with ADD love to climb all over adults. You leaning into them robs them of that pleasure.