My dad only buys 8' bed trucks for the farm. Thinks its a must have. I don't agree.
I was really wanting to get a 6'4" bed, and was really concerned that a 5'7" bed with ramboxes would be too short after owning my 3rd gen (6'3" i think?)
Anyway, i can say after getting the 5'7" bed that it really has not been a problem at all, especially with the factory bed extender on the rambox and the cheapo hitch mounted bed extender i keep in the rambox.
Anything up to 12' can be hauled with any of the bed lengths. I would recommend hitch mounted bed extender though for 5'7" beds (supports ends of the load and has reflectors) at 12'. 10' is not a problem without it..
Anything over 14' i'd want to haul on a trailer, regardless of the bed length. I hauled 16' several times with my 3rd gen and the hitch mounted bed extender but really wouldn't recommend it. And trailers are cheap.
There have been very, very few times that the 8' bed in my dads trucks where really nice. If you want to haul a fuel tank or toolbox or pull a lot of gooseneck trailers or need the extra cargo then i could see wanting one. But there have been many more times where the worse maneuverability or the worse break over angle have caused issues, so i don't think 8' is worth it (especially not crew cab 8').
To each their own.