I just watched ... well, I shan't name names, but it was a brief series of videos on the "important basics" of Photoshop. It started well - Layers. Yes, layers are critically important, no argument there. Opacity. Ok, I guess opacity is important, in a way. Masking. Yes! Masking is damned important, though most people don't use it intuitively, in my opinion. (Article: The right way to use layer masks)
Number 4 - Blend Modes.
C'mon, give me a break!
I see the use of layer blend modes in post-processing very often on the "beginner" forums I frequent. I suspect the author of the aforementioned videos is largely responsible for this.
Blend modes are not for enhancing images. Try talking to a pro photographer, or going to the RetouchPro forum (where the real experts hang out) and mentioning blend modes. If you're lucky, you'll be laughed at politely, and gently pointed towards Levels and Curves tutorials. If they're in a mood, you'll be flamed mercilessly.
I understand why they're popular among JSOs. They give a quick "wow!" to your photos. Without examining too closely, they appear to improve your photos. They even appear to give you control over your editing.
It's an illusion.
Blend modes give buggerall control, and can't be relied upon to improve your images under close scrutiny. A Screen layer lightens shadows, but blows out highlights. A Multiply layer darkens light areas, but it clips shadows. An Overlay/Soft Light layer creates midtone contrast, sure, but it brightens highlights and darkens shadows without control; and it does it uniformly, which is rarely desirable.
No, ditch them. Take genuine control. Read histograms, use Levels or Curves, master selections. You'll never regret it.