It depends on how well you want things done. Convolution with real HRTF > approximative IIR or FIR of a HRTF > normal cross-feed. It's also a question of diminishing returns. Normal cross-feed takes you to 80 %, approximative filters to 90 % and convolution with real HRTF to 95 % (numbers illustrate the principle). If 90 % is enough, why isn't 80 % as well? Huh.