In my case, i went to ZBroz spindles, anyway. So not sure if required or not.
However, check out the factory 36" front end. It does NOT include spindles, nor does it say you need to buy separately. Ref PN 2883686-309. That kit was specifically made to take a 16-18 to the same narrow front end of a 19. The instructions reference installing the "previously removed spindles". If there is some other benefit you might see by using '19 spindles, that's a different story. Would need to dig for some commentary about the benefits of the different spindle design and see if it makes sense over being marketing hype. But are you required to change spindles, no.
Any after market companies that tell you are required to change them, are just looking to sell you "their answer" to the issue. I.e. sell more stuff, whether you need it (its required) or not.
IMO - While there are some factory things that make me shake my head, when it comes to geometry - I think they are pretty smart. Looking at the factory accessories tells me most of what I need.