I am doing exercises from Fun With A Pencil and now I have to draw spheres
And now I have a question - am I supposed to draw them in orthographic projection or in perspective? For example if I am to draw the 2nd sphere in orthographic projection, distances between the axis and the curvse next to it are supposed to be the same, but if it's in perspective projection, distance between the axis and the dotted line will be smaller than the distance between the axis and the line to the left from it.
Disclaimer: I don't own any of these images. I just found them using Google. Please don't sue me. I am poor.
I would say, if you want to go for the classic Disney or realistic look, then use perspective for sure. And a bit more than what you should for a portrait. Take a look at these Mickey mouse heads and the placement of the eyes. They are definitely in perspective.
Cartooning, though, is closer to art than to design. You can deviate from a strict set of guidelines and take artistic licenses to express yourself. Cartoon designs are known in particular for distorting perspective just to add drama to the final result, sometimes in very creative ways.
Having said that, the head of a character is usually not very big so you could decide not to use perspective to draw its the features at all. The head, in the "real world", will always be perceived in perspective, of course, since perspective and foreshortening are artifacts of our own eyes, but they are not that noticeable on facial features or small objects. Take a look at Garfield's eyes in this image, for example. They are almost in an orthographic projection.
If you are drawing a head that is very close to the eye of the viewer (so it looks very big) or the head of a huge character (say an enormous robot that takes a whole block) then you could project the facial features in perspective in quite an exaggerated way to convey that. Still, it is your choice. Take a look at Adventure Time designs, for example, where perspective projection seems to appear almost randomly.
Another final comment: I would not expect the sketches from "Fun with a Pencil" to be accurate draftsman's renderings. Take a look at these two sketches, for example. Notice how the placement of the left eye (the one away from us) in relation to the vertical guideline is different in both cases. I don't think accurate perspective was one the writer's goals, but just (awesome) general guidelines.
In that particular exercise, in my opinion... Dosen't matter.
Lets say the eyes are the feature where you want to mark perspective. Probably you think that you need to make an eye smaller than the other. But in a cartoon the diferent eye size can be a "hum" expression, surprise, or a specific feature of a character, a pirate, a mad scientist.
Play with the expression, don't worry about the perspective. You need to mantain the eye axes, yes to mark rotation, but not a strict perspective.
You need perspective in the head where you need to exagerate dimensions, for example it is a supervillian reciving a blow from the hero and you are making a dramathic close up frame; or you have a huge sculpture of an ancient god.
Besides that, a normal framing will have more perspective in the body, not in the head.
The short answer is "yes."
It seems like it is obvious from your question that you know the difference between a circle in perspective and an ellipse. This difference is something I had to spend about a month on in Freshman Drawing, but the point there was to learn how to translate what I saw to something as faithful as possible.
However, in the context of the question, this is one of those things where there is no "supposed to:" you are drawing cartoons after all.
There is a mantra in music that says "practice doesn't make perfect, perfect practice makes perfect." This says you must use accurate perspective.
The corollary to this is "imperfect practice makes style." This says you may do whatever you want.
One of the masters, in my opinion, of cartoon art is Bill Watterson. Part of his genius was his blend of both choices.
