There's actually a range, from toes forward up to 45 degrees out, depending on your leg structure, size/shape of the horse's barrel, etc. Any more then 45 degrees and you're in danger of riding with your heels in your horse's side off the back of your calf. That's bad because 1) you'll constantly be driving your horse forward and 2) your leg will not be as secure as it could be. I will often remind my kids "toes forward" because it gets a better response then "put your toe closer to a 45 degree angle..." They also get the "why your leg goes in that particular position" lecture quite a bit so they understand that there's more to it then just "toes forward".
NJ- it's not necessarily a "toe" issue, as it is an entire leg issue. If your toes are out, your entire leg is most likely not in the best place. There is a particular position where you will be the most balanced and secure, while being in a place to use your body the most effectively. For a few people it is physically impossible, For some it's physically really difficult, for lots it's completely unnatural, but can be taught. I used to have the habit of riding off the back of my calf. I was pretty good at it. Pretty secure and got a decent response from my horses. It felt horrid to change my leg, but the trainer I was working with made me. After lots and LOTS of work, my leg is in a better place and it's even more secure then it used to be.