Cropping PCM sounds in your wave editor on zero-crossing does not guarantee that transitions in Wwise will be glitch-free, because once they are converted, there is no guarantee that these zero-crossings are preserved.
Sample rate conversion moves samples (along the time axis) of the orignal wave, so you cannot be sure that there will be a sample at zero where there once was.
Frequency based codecs use FFT windowing processes that don't guarantee phase. Vorbis and XMA are two good examples. A square wave might not look like a square wave once decoded. It only sounds the same because the amplitude of its partials are preserved.
Usually, this is unnoticeable with normal sounds, even in a continuous container with sample-accurate transitions. You may hear artifacts if you use pure tones though.
In the Segment Editor, waves are displayed at 48 KHz, so there is no guarantee that the last sample of your carefully trimmed audio clip will fall on a zero-crossing at run-time. Likewise, placing your exit cue on a zero-crossing may sound well in the authoring tool, but not as well in game. If that causes a problem, use a small cross-fade in your music transitions.
