If you plan on installing the solution in a closed network, e.g. if the customer has one physical location and there is no outside access to the NAV web service, then it is not necessary.
If you decide to open up the web service for the outside, then setting up SSL is something that must be careful considered. It is not a given that anyone is trying to gain access to the customers data or is interested in gaining access to the customers systems. But if you are not using SSL the data can be read as plain text and this includes the user credentials used by SHOPFLOOR to pass to the NAV web service.
NAVEKSA recommends that you as a consulting agency always advise your customers about the pros and cons of setting up SSL and leave the decision to the customer.