Cristofori didn't invent the piano in a vacuum. He invented the instrument because there was a demand among composers and keyboard players for an instrument that could make dynamic changes through touch - not just by changing registers or manuals.
What that tells me is Bach and other composers - and players - thought this would be a good thing, and would make full use of it. Also, the clavichord did have the ability to vary dynamics by way of touch - however the dynamic range is probably pp to mp. Not only that, you could execute vibrato on a clavichord.
Bachs music is primarily contrapuntal, and it sounds ridiculous if you play every voice at the same dynamic, and the entire piece at the same dynamic. When playing a fugue, you constanty have to consider which voice(s) to bring out more. In addition to that, you have to use dynamics to shape each phrase.
That said, you have to be careful in following the dynamics in certain editions of Bach - especially Czerny, which you should not use anyway - because they may not be stylistically correct.
So, the answer is: you always have to use dynamic contrast in Bach, but you don't always have to follow a particular edition.
BTW, it is a misconception that Bach never added dynamics to his keyboard music. There are several examples where he did.