Accessories - Housewares - Finishing Touches - Hanging light pinctures with picture hook -- problem with angle of nail?

I have recently hanged a number of relatively light pictures (the heaviest was way less than two pounds). I used picture hooks like the one in this link, rated "for 10 pounds":

http://www.doityourself.com/invt/u893792

For the lighter pictures, I used one hook; for the heavier picture, I used two. I have plaster walls in good condition.

However, I didn't drive the nail at a 45 degree angle; instead, I drove then "flat" (at a zero angle) or "almost flat" at best. For a couple of pictures, the nail actually went at a extremely low "negative" angle. But so far, all pictures appear to be securely hanged.

Is the fact that I didn't drive the nails at a positive angle a problem? I mean, these nails seem small (less than one inch) and I still don't understand the mechanics of these hooks hodling up to 10 pounds. If the lack of angle is a problem, and if there is a risk of pictures suddenly falling, should I redo this?

Thanks

The angle is what makes the hangers not come out of the wall. Picture yourself hanging from a ledge with your hands at a 90 degree or at a 45 degree angle. Which one do you think will allow you to hold on better?