Yes, you may use only NZ@McD1 for now bacause of the following reasons:
1. after 9.0 firmware update streetpass are no longer works with password-protected attwifi nintendo zone ssid, while it works fine with NZ@McD1.
2. Nintendo spreads streetpass data for NZ@McD1 SSID (or maybe others wuropean too) with no MAC address lookup, so you can get a streetpass even without changing MAC - this won't work with attwifi (well, except streetpass weekends, which is the current weecked luckily)
3. Windows drivers prevents setting the real hardware MACs, so you were never able to actually reach the real attwifi nintendo zones (well except the very small number of wireless adapters which can actually set the real MAC but not in a straight way, whch is not used by any existing mac change method for now - I'm still testing), all the streetpasses you've got before was from the similar users or from the streetpass weekend streetpass sharing.
Thank you, I knew about the win drivers and that we weren't really reaching the NZones. This answer clears completely my doubts about the attwifi issue .