Actually, when it comes to saving .wav files in UTAU-Synth, you don't even have to listen to the .ust file before hand. ouo
Simply open the .ust, fix it up a bit if needed, and then either go to the file menu and select the (R) option or hit "command + r" on the keyboard, minus the quotes and "+." UTAU-Synth will then ask you to name the .wav file and save it. Click save and UTAU-Synth will begin rendering the finished product and will notify you when finished. It's a lot quicker than in UTAU and it's great if you want to make a lot of last minute corrections like me. =u=;;