I fill from the mouthpiece end also, and haven't tried from the battery end, mainly because it seems like a pain to wipe off the excess juice. (I have been really lucky about not 'bubbling up' when filling from the mouth piece so far, but even when I do get a blip, it's a tiny bit and easily wiped out.)
Thinking about it though, people often use the Taryn Spin or similar centrifugal force to try to make more juice move towards the battery end. Therefore, it isn't just 'wicking it up' efficiently on its' own. If you fill from the battery end, it soaks into the batting and is already saturating the end closest to the atty.
Just a thought... I bet there are a million perspectives on this one!