Calves are a little unique. They take a lot of abuse in day to day life. When working your calves in the gym, they recover very quickly and in turn, it is difficult to fatigue them like other muscle groups. I'm a big fan of doing multiple sets of 10-15 reps and going as heavy as possible. I also try to rest only 30 seconds between sets.
You can vary the angle of your toes, but be aware of your knees. For example, if you point your toes in, try not to let the angle of your knees change, don't let them sag toward the middle. For toes-in calves, keep a wide-ish stance and keep knees strong.
It is important to change the angle from time to time to avoid disproportionate calf growth. I developed a training injury as a result of one side of my calves becoming stronger than the other and it eventually resulted in a stress fracture. So I prefer to change up 1-2 sets on each leg day (contrary to yakubu's advice). Let's say I'm planning to do 4 sets of 12 calf raises.. I might do 3 sets with straight toes and 1 set turned out or in, depending on mood.
Great question.