What is the highest relevant stopband attenuation for a 1-D digital filter?
That really depends on the application. For example this processor: http://www.analog.com/en/processors-dsp/sharc/adsp-21364/products/product.html
has 140 dB SNR in it's sample rate converter. That means the stop band attenuation of the interpolation filter must be at least as high but it's more likely to be 150dB - 160dB. You need extra head room to manage the fractional indices into the poly-phase filter bank in irrational sample rate converters