Usually, people are aware that they have it, but don't want it to be so pronounced.

If you can use Gimp, there is a plugin called Wavelet decompose.

In this site there is some information on how to use it to retouch pictures.

I like that technique because you don't need to complete remove what doesn't please you, but you can minimize it so the picture still looks natural but without all that impact of some acne or so on.

Edit

I don't know how your models look like, so I took one example from wikipedia that, by it's turn, took it from some US government site.

What the plugin does: Basically it separates the image in many layers, and each layer contains some information from the original image. Each layer has some detail level, allowing you to edit each layer/level independently of the others. For example, color will be on one layer, small spots will be on another.

Below, I think you can guess which one was the treated image. I used that plugin to decompose the image in 6 layers, and edited just 2 of them (painting what I wanted to change with neutral 127,127,127 gray.

It was a very fast job. It took less time than the following picture, that anyone can see was done very lazily.

Here you can see the original picture (1), and it decomposed in 6 frequencies. The (2) is the residual image, where you have mostly the colors. (3) to (8) are the decomposition, with increasing frequency (so in (3) you have less detail and in (8) you have the highest - finer - detail).

You can see that (4) and (5) have great variation of color, so editing them reduced the acne/scars. The post-processed image still have acne and so on, but very lighter, still making the model look a real person with skin imperfections but not so strong.