It does not sound like a big issue but user interface wise it is. A Drop down listbox only takes existing values as a Combo box allows entering of new, unknown, values. When talking about Combo boxes don't forget to clarify what should happen if the user enters a value that doesn't exist in the list.
"We would like to select the Customer from a Combo box."
"What should happen if the user enters the name of a Customer that doesn't exist?"
"The Customer should be created."