Yes, you could just leave the iptables alone if you just want to connect to N900 without a tethering need. And you don't need that modprobe ipt_MASQUERADE as well. However, one thing you must note that if you're using Windows PC to connect to any WiFi, it always tries to make it as the default route, i.e. all internet traffice request would go to WiFi. Unless you made change to this default behaviour, you might not be able to go to internet via your private network of your PC when you're connecting to WiFi.