No, you do not need to add line items to a Job Type, however, by adding the line items it will allow you to generate an invoice when you create a job quickly.
Handy Hint; if you have various pricing for the same type of job it may be easier to have no line item on the job type and add the invoice to the job as you create it. For example if you do an inspection and the price varies depending on the size if the house then having a single job type of 'Inspection' with no line items would be easier than having three job types for a small, medium or large property. It also makes it easier to pick the right Job Type when there are fewer options to choose from.