To answer this question, you first have to distinguish two things: power flatness and OSNR flatness.
Power flatness (sometimes called balancing channels) refers to the maximum power difference between channels. This value is important because it indicates that all channels are amplified by roughly the same gain at each optical amplifier. If one channel is much more powerful than the others, the gain might be uneven. A typical figure for power flatness is max 2 dB.
OSNR flatness is a different story. Even when the OSNR is very different from one channel to the next, the system will work just fine. For instance, if channel 1 has a 30 dB OSNR and channel 2 has a 20 dB OSNR, the system will work well as long as the power flatness is good (. What matters is that each channel has an OSNR value above a specific threshold, which is usually greater than 15 to 18 dB depending on the system.
Moreover, using the right OSNR method is essential. The IEC method works for 10G without ROADMs. However, as soon as 40G noncoherent or ROADMs are introduced, an in-band OSNR method must be used. For more information on in-band OSNR, be sure to read the Optical Spectrum Analyzers in Next-Gen Networks white paper.