I know I'm really late to answering this question, but both of the answers here are wrong. Its not actually faster to grab the bottom, its faster to grab it at the top. It turns out that mario spends as much time on the flag pole no matter where you grab it. The difference comes from the acceleration in the air versus on the ground. When you grab at the very top of the flag pole, mario doesn't hit the little block on the way down and accelerates faster than if he walks on the block. This small speed difference is enough to save a frame rule.
For proof, watch the following video.
For more information on frame rules, you can read this article http://tasvideos.org/GameResources/NES/SuperMarioBros.html#The21FrameRule A short summary of what that says is during the black out transition from one level to the next, the game actually waits up to an additional 21 frames. I'm not sure the reason the game does that, but it means that often in a level, saving a single frame from better execution won't save any time because the game will then spend an additional frame in the next loading screen. Usually only frames saved on 8-4 matter because of this.
The video also talks about using the flagpole glitch. This glitch cuts out the flag lowering animation, which makes it faster than grabbing the flagpole at the top. This glitch previously was thought to be too precise to be usable by live speedrunners. Using a new setup found recently, the glitch is now possible to perform which saves a frame rule (meaning the world record is being challenged again!)