  We present elastic and inelastic spin-changing cross sections for cold and
ultracold NH($X\,^3\Sigma^-$) + NH($X\,^3\Sigma^-$) collisions, obtained from
full quantum scattering calculations on an accurate \textit{ab initio} quintet
potential-energy surface. Although we consider only collisions in zero field,
we focus on the cross sections relevant for magnetic trapping experiments. It
is shown that evaporative cooling of both fermionic $^{14}$NH and bosonic
$^{15}$NH is likely to be successful for hyperfine states that allow for s-wave
collisions. The calculated cross sections are very sensitive to the details of
the interaction potential, due to the presence of (quasi-)bound state
resonances. The remaining inaccuracy of the \textit{ab initio} potential-energy
surface therefore gives rise to an uncertainty in the numerical cross-section
values. However, based on a sampling of the uncertainty range of the \textit{ab
initio} calculations, we conclude that the exact potential is likely to be such
that the elastic-to-inelastic cross-section ratio is sufficiently large to
achieve efficient evaporative cooling. This likelihood is only weakly dependent
on the size of the channel basis set used in the scattering calculations.
