For decades my approach has been to establish terms and pricing up front, then work with whatever pay system is in place with the client. Depending on whether it's a small company or a big corporation, getting paid can be a different scenario. For instance: TV networks can be a barrel of red tape to work thru at first, but once you've past that phase, and are in their system, getting paid is pretty streamlined. From my experience, big corporations tend to have the slowest pay systems. Small shops may need to collect from their client before being able to pay you for your services. Ad agencies may need to issue a PO for the job before you send your invoice. And some prefer to receive just one invoice at the end of their month when they close out accounts and projects.
I try to be flexible and work with the clients' pay systems. That said, my invoice states '15 days net please',(from invoice date) which usually gets me paid fairly quickly once the invoice goes out, and repeat clients honor that request as best they can. The main goal is to build a working relationship and acquire clients for life. So being flexible is always beneficial.