I'm going to go over the various expressions the Pixel Norns can use and any differences they may have. If there are any missing frames for any of the expressions (if you turn the complete set of sprites into a real breed at any point) you can use the next highest or lowest head 'looking' sprite in that set. You can't tell which direction a Pixel Norn is looking, other than if it's looking left or right, when it blinks. This shouldn't be a problem though, because Norns blink for a split second.
Happy: This expression's conditions are unaltered.
Neutral: This expression's conditions are unaltered.
Sad: Pixel Norns only use this expression when they have high amounts of punishment, and even then, only if the Hand is in their line of sight. So, even though you can't tell if the head is looking up or down, when the Norn's head is using a side view sprite, it shouldn't be a major problem unless your Norns are very disobedient.
About the sloppy placement of sprites for expressions beyond the first two: I realized that there were no 'facing left' sprites for any of the expressions beyond the first two as I was about to choose the image, so they were thrown together at the last second.