Why deploy and set up JustOn in a production org?
Generally, we recommend to deploy and set up JustOn Billing & Invoice Management in your production org. There are several reasons to do so that outweigh the potential security gain of a sandbox.
(1) Installing and setting up JustOn in your production org has no impact on your running business processes in Salesforce. JustOn ships a number of new objects, Apex classes, etc. that (initially) do not interfere with your org and other installed apps in any way. Think of a new app you install on your phone - it is completely separate and does not affect the rest of your phone's functionality. Even better: as long as your users are not assigned a JustOn license, they do not see anything of it.
(2) A great part of JustOn's configuration is implemented in actual data records, like custom setting records, invoice templates, master subscriptions, etc. That is, change sets, the usual mechanism for deploying changes between Salesforce orgs, do not work for this kind of information.
(3) Sandboxes usually do not reproduce the same permission and role setup as a production org. That is, configuring JustOn with a possibly limited set of permissions and roles may cause errors or malfunctions in the real environment, which requires another time-consuming configuration in the production org.
(4) Setting up JustOn in its real "run-time environment" (the production org) saves a lot of time, since there is no need to create, upload and deploy change sets, to repeat configurations that cannot be transferred, etc. For many businesses, the time gained this way outweighs the (potential) advantage of configuring the new app in a safe org.
This advice does not relate to custom development or functional tests. To this end, we still recommend to create a sandbox copy of your production org after JustOn is set up.
So the recommended setup path is
Production org: Deploy and set up JustOn
Sandbox org: Functional tests and custom development
Production org: Roll out custom development and possible adjustments