I'm from the "spread it out" school with moderation.
I do try to control what material gets written to which drive, but not to the extent that chicken eggs are always in basket 1 and duck eggs are in basket 2 and 3 etc.
[*]The main consideration for me is that if I have a failed drive that is somehow not recoverable by Parity (or 2+ failed drives), trying to recover 4TB manually, in my opinion, is a lot less painful (and easier to succeed) than trying to recover 8TB manually (despite the 50% probability of not having to recover anything as the failed drive is empty).
[*]If the drive is completely not recoverable, losing 50% of movies and 50% of TV shows, in my opinion, is a lot less painful than losing ALL the TV shows or ALL the movies.
[*]I don't have the luxury of buying drives to leave them empty.
The "moderation" part is the fact that I don't strive to make sure each drive has the same amount of free space. Close enough is fine, but not exactly the same. I basically just let unRAID decides based on a "Most Free" distribution.