It doesn't really matter where in the lower sections you are. It's just important because sometimes there are security checking you actually have a ticket to the lower sections. Otherwise you won't be allowed to go down there to dance and jump around.
And for those interested; everyone who bought tickets from Arcade Fire.com (CrowdSurge) will get a ticket to the lower sections from what I've gathered. The very first ended up in "Section C Lower" (1) and when that section was full they started filling up "Section B Lower" (2).
This is awesome because it means it will pretty much only be people in the lower sections who truly love and adore Arcade Fire. Bring it on!!