Just because someone at a pet store recommends a product, doesn't always mean it is safe to use.
I'd recommend just using the pine shavings or aspen shavings.
I have used pine for 17 years with my chinchillas and never had a problem.
If you do a search for corn cob bedding and chinchillas on google, you will find several websites that say that corn cob bedding is not safe for chinchillas. It's okay for other pets but not them. The pine or aspen shavings won't cause fungus if you clean the cage regularly - just like any other type of bedding for pets. Fungus itself can come from a variety of circumstances, whether it is other pets transferring the spores, or bacteria growth in a dirty cage, or even you transfering fungus spores from your hands onto them. Moist areas of the chinchilla's body is where fungus will most likely show up at first (eyes, nose, private area...). Hay can even pass on fungus spores. It's all about having a clean environment in order to prevent fungus. Pine shavings or any sort of bedding, itself, won't cause fungus if the cage is kept clean.