If you go to Maintain > Customers, select a customer, and enter a sales rep on the Sales Info tab, that sales rep ID will automatically be entered on sales orders for that customer. When the order is invoiced the sales rep will carry over from the order to the invoice. That also means that if the sales rep was left blank on the order it will be blank on the invoice although you can still enter a sales rep on the invoice.

If you don't want to assign a sales rep to a customer, you can leave that field blank in Maintain Customers and manually enter it on the sales order.