Hey, I'm Bunbun from the forum link posted earlier. ^^
Unfortunately, we haven't got a reason as to why we can only find people on the PAL version and not the NTSC-U version yet.
I also experience random 81010, 91010, (dunno about 81310) errors when searching in both NTSC-U and PAL versions after a while of searching (half an hour to 3 hours). In the PAL version, when I find someone (which takes forever because I don't think there's many PAL users searching in basic brawl) we usually disconnect with a 91010 error after a few matches and this has also happened when playing against friends. I shrugged this off because I just assumed it will be fixed sometime in the future as the servers are still under development. So basically, we're just waiting for a response from Ciapa (from that forum). Me and another person from that forum dumped some packets for him. Hopefully he can figure out what's going on.
If you were able to play basic brawl/team battle on the official servers without forwarding ports (or didn't experience any times where you couldn't connect to a friend) then you're probably fine. But just to be safe, it wouldn't hurt to port forward. xP Easiest way is to just set your Wii to DMZ in your router.