It should depend on what is defined as "manga" -- does any type of Japanese-produced, Japanese-published cartooning go, regardless of style/tradition, production language, etc? From Sazae-san and similar gag manga to mainstream shoujo and shounen to the Marvel Comics clone I bet someone in Japan has pooped out at some point?
Personally, I would suggest the following guidelines for webcomics:
It's okay if:
* it's created by a (Japanese) author who has had works published in print (in Japan etc.)
* or failing that, it's has been published on the Net or in print by a Japanese manga publishing establishment