Hardened or unhardened doesn't make a difference in stiffness. What you want to know is how much the rollers bend under load. Hard to say how much is allowable. The more it bends, the less accurate your work. Maybe something in the ballpark of 0.1mm for you setup?
The bending is calculated (in this case. supported in bearings on both ends, load evenly distributed):
f = 5 * q * l^4 / (348 * E * I)
With: q: load in [N] l: lenght in [mm] E: elasticity modulus in [N / mm^2] for steel it is about 2.1E5 [N / mm^2] I: I think you call that sectional momentum in [mm^4]
For a round bar, I is calculated: I = pi * d^4 / 64
with: pi: 3.14 :-) d: diameter in [mm]
HTH, Nick