Do I need to allow free space as an over provision on MacBook Pro's built-in SSD? I have a 2017 15" MacBook Pro with a built-in 1TB SSD.

As I understand, with SSDs, the best practice is to leave a portion of them free as an "over provision". 

Do I need to do this with the built-in SSD in the MacBook Pro? or has Apple already accounted for this in giving me 1TB of space? e.g. There is actually something like 1.1TB of space, but the firmware keeps that additional 0.1TB free as an over provision.
No.
Over-provisioning is transparent to the user, and done at the factory. SSDs normally have more size than can actually be used. Over-provisioning is not something that the user needs to do.

Having said that, SSDs still need to maintain spare space within the volume, in order to have some space to accommodate working files. Between 10 and 15% is usually recommended. Some say as much as 25%.