Yes. We're talking about security on two (almost) independent layers. WLAN encryption secures all data transfered over the air (but no further). SSH encrypts all data transfered through it's connection with the server (except maybe the key exchange, but this is done in a secure fashion too). So if you start a SSH session when on an encrypted wireless network, the data is actually encrypted twice, once by SSH, and again by the wireless connection. Otherwise it's encrypted only once, by SSH.