The airave gives you a strong 2G signal, not 3G (unless you have the newest version), so by leaving wifi on you are still using the wifi radio, not the 3G radio, so you're still saving battery. Even if you have the 3G airave you still save battery, though, because the phone is still seeing 3G so it still refreshes the network connection more than it does on wifi.
Unless you're really running short on battery life, I wouldn't bother turning off the 3G radio. If both radios are on and there is any wi-fi signal, the phone uses that for data and doesn't use the 3G radio.
When I first got my Evo, I was switching the radios on or off depending on where I was and what signal I had available. I got tired of that really quickly and, after reading some informative posts, I decided to just leave both of them on - if my battery life is any different, I can't tell.
You might want to try it both ways to see if it does make a difference in your situation - everyone uses their phone differently.