For me, aperture isn't a priority because that varies from lens to lens..but usually to get the sharpest of any lens, it's usually 2 stops over the max and using 5.6-8 usually works for all lenses.
When i do portraits, i look out more for:
1) Position and framing
The camera settings aren't a priority.. you can have the best lens at the best aperture with the most optimal setting but if you subjects are in a place with an ugly background, messy background or places in a bad frame, the photo will ultimately still look bad..
Even if you shot at the wrong aperture and your subjects are the sharpest, if the photo tells a story and directs the viewers eyes to the subjects instead of the background, you will still have a good photo.
When i approach a shoot, i always tell myself i'm using a PnS camera, i will choose a location first, then position the people, then frame the shot, then i start to tinker with the camera settings..always remember, the photo is the one telling the story, not the camera setting
There are camera settings that can be worried later but aperture isn't one. Aperture is important because it also defines your subject and framing. It is also an important part in determining how strong your light (strobist stuffs) needs to be (though this appears just to be secondary to DOF). As you have said, it's important to direct the viewers to the eye of the subjects but what if your DOF is too shallow and you weren't able to render the eye properly? Aperture will also define how much background you want to blur or not which is also significant to framing. So yes, you will have to worry of your camera setting during your positioning and framing at least when it comes to aperture along with the other 2 elements of exposure (shutter speed and ISO). The rest of the camera settings like WB, picture styles, etc..., you can worry during PP if you shot raw. So shooting raw is another advice most professionals can give as it allows you to set other camera parameters during PP.