I stumbled upon two videos while on youtube:
http://www.youtube.com/watch?v=XBpiSTYo0MM -NES
http://www.youtube.com/watch?v=F16oE_qAWOA -SNES
The first one shows Mario walking into the base of the flagpole and his comments say that he did this by killing a bullet bill.
I then found another video showing the same glitch on the all-stars version in the second link.
I thought that even though it looks like an inconvenient glitch to pull off, I think it could save time.
It appears that right after performing the glitch, Mario is sent backwards off the flagpole and immediately starts walking into the base of it (The left side), and Mario cannot progress any farther. The countdown on the timer begins right as Mario is stopped, which is almost immediately, compared to the current TAS where Mario has to walk to the castle door and then the countdown of the timer begins.
I think if there was ever a case where an enemy was very close to the end this could save time, but I also think there could be a way to pull this off without the use of an enemy. If Mario can be forced to the left side of the flagpole after doing the flagpole glitch, I think it will result in a faster time.