I'd prefer my brakes to be more than enough for the job, rather than "good enough".
There will always come a situation where you're going to need good, reliable, powerful brakes! A larger rotor means better cooling properties (so less chance of fade) which in turn leads to generally better control over braking and the bike in general.
I always used to fit the biggest rotors I could get away with on all my bikes, whether they were for XC, Enduro or DH duties. Admittedly, you don't need a 203 rotor on the rear, but I ran my Cove Stiffee with a 203/203 Mono M4 combo. There is something very reassuring about knowing that you have enough braking power for any situation, and the small weight penalty was well worth it.
I ran a 238/203 combo on my Intense M1
- Hope had to send me a custom size rotor for the front brake as I was running an early set of 2003 Monster Ts which had a really odd mount size! I remember sticking the 203 rotor on and it not even coming close to the pads!