While ultrasonication is a widely used standard method to disperse nanoparticles across scientific and technological communities, in academia and industry, a parameter that has received surprisingly little attention is the particle mass fraction $W_s$ at which sonication takes place. For anisometric nanoparticles like nanorods, nanoplatelets and nanotubes, its importance goes far beyond viscosity tuning, since a dilute isotropic suspension of anisometric nanoparticles transitions to a liquid crystal (LC) phase if the particle content surpasses a threshold, via a wide biphasic window of isotropic–LC phase coexistence. By systematically varying $W_s$ of suspensions of cellulose nanocrystals (CNCs) across and beyond the biphasic range we show that the particle mass fraction has critical influence. The optimum is near the stability limit of the isotropic phase,  where the sonication shear flow induces a temporarily ordered paranematic state throughout the sample which assists particle individualization and minimizes damage. We confirm by atomic force microscopy that sonication at this particle mass fraction yields the longest and most slender individualized nanorods and we see significant impact of $W_s$ also on macroscopic properties like the phase diagram and the pitch of the cholesteric LC helix into which CNCs self-assemble. Our general conclusions apply also to other nanorod, nanotube and nanoplatelet suspensions. Many reproducibility problems common in work with such nanoparticles may be due to the insufficient attention to the choice of $W_s$.