In my limited and recent hunting experience, I used what shot best in my rifles and aimed for the head. For me that ended up being Federal standard velocity 40gr lrn out of my cz 455 lux and CCI 40gr cprn out of my stevens 300f trapper. These seem to do the job on grouse and squirrels. In thick woods the grouse seem to not place the direction of my shot and I can usually get off at least a second shot if I need it.
As a side note, I've noticed in my war against the squirrels on a friends property that they seem to undergo far less vigorous death throes when head shot with round nose compared to hollow points. They also seem to flop more vigorously the greater the velocity of ammo... CCI Stingers being the "floppiest" ammo. Anyone seen this effect before?