Here's some links that you can read that might help you determine what is causing the cough and whether a vet appointment needs to be made.
http://www.glassyorkies.homestead.com/ Select hypoglycemia on the left and then collapsed trachea. This will discuss reverse sneezing vs. collapsed trachea.
Also
www.canismajor.com/dog/cough.html
This is other conditions that may cause a cough.