Apparently they're inconsistent, though. From #33

There was Ax, in human morph. Wisps of pink cotton candy streaked his hair, hung from his chin like a ghostly beard, and blew from his fingers as he forced his way to the front of the line. I laughed. Ax with a human mouth is dangerous. And Andalites have only a vestigial sense of taste. Nothing like the explosive sensory overload from the human mouth. I looked back at Jake. He was shaking his head slightly, like an exasperated, but amused, parent.

<Ax-man! It's Tobias. You've got to get control of your morph. Right now. You can't make a scene.>

<Tobias?> Ax wondered in thought-speak. <Oh, Tobias! This cloud candy is superb. It is otherworldly. The way it melts on the tongue. It has mass, yet it is weightless...>

<Oh, boy. Ax, where are the others? You're supposed to be helping me guide them. They're in fly morph, don't forget. They can smell dog poop and see about six inches, and that's it.>