In home networks, the idea of merging with someone else's network is... most certainly not worth worrying about. Maybe you marry someone or become roommates with someone who also picked fd00::/8? And you still want two separate subnets? Other than that I don't see a scenario where it matters.

Granted, if you're doing this in a corporate setting (where merging with someone else's address space is a lot more realistic), then yes definitely pick a random 40 bits. But at home? Who cares. Same as using 192.168.1.0/24 instead of a random 10.0.0.0/24 subnet... it's not worth worrying about.