If the setup can be assumed to be equal (which is not a safe assumption) and the larger rotors will fit your frame or fork, then yes, it is always an upgrade. You will benefit from added leverage against the movement of the wheel (80mm lever to 90 mm lever, so 1/8 increase), and a similar increase in rotor area across which heat can dissipate.

However, this is not to say it is at all necessary - I have weighed up to ~270 lbs and never found my Shimao hydraulic calipers and 160mm rotors to be at all insufficient, even on steep and nasty off road trails and longish descents. And, as others have mentioned, the manufacturer of the frame and fork may recommend against it.