The criteria is based on your blocking signals. For example if you have an interfering signal at 3 MHz offset to your operating frequency then it makes sense to measure your phase noise at 3 MHz offset. The level of PN required depends on the required SNR. For the downconversion mixer, the desired signal mixes with fLO and the interferer at f+df mixes with the phase noise at LO+df. So they end up converting to the same frequency. I think this equation is correct:

L(df) = Sensitivity + 3 dB - Pinterferrer,df - 10log(BW) - SNRmin

L(df) is phase noise at offset frequency f. Sensitivity is your minimum detectable signal level, Pinterferrer,df is the interferer power at offset f, BW is bandwidth and SNRmin is your minimum required SNR at the input to the ADC.

I think a lot of publications on VCOs don't follow a particular standard, but just try and get the best phase noise. So for comparison purposes they just chose an offset of 1 MHz or whatever.

cheers,
Aaron