chain transmission without tensioning

[Dival]

I am now working on CADing a 6WD model. I want to use chain transmission without tensioning. From your experience, is it possible, given a high capability of CNC machining for the drive base channels?
Also, I was wandering what tolerance I have while designing it. Do I have to make the distance between the sprockets exactly the expected length? A little bit less? Is 1 mm deviation, for example, enough to make the chain fall, or make it be over-stretched?
I am particularly interested in using a 25 chain, but any answer that deals with 35 chain or belt drive will also be very helpful to me (other members of the team are now designing belt drives).