I suppose there could be a time and a place for the head shot (under an ideal situation), but realistically that shot should be avoided B/C
1) it's tricky
2) Safaris cost a lot
3) time is limited
4) you don't want to end up "paying for" or much less "tracking" a buff that the horn or bottom jaw got shot off of. Mistakes like that literally take days away from other hunting you could be doing.

Unless he's feet away and bearing down on you (very unlikely situation) there are just too many variables to any given "frontal shot" situation to try for the brain.

You may be able to do it and pull it off (again, in an ideal situation), but the frontal chest shot is a much safer or rather "more sure" bet/alternative. More vitals to hit, bigger target and more certain of doing "trackable" / "killing" damage to a buff.