As people before me already said, it does depend on what the sheet is intended for.
If it's a model sheet for a 3D artist, it usually depends on the 3D person if they need front/back/side or can work with 3/4 view, or want something else entirely (e.g. I once had to create a rough top view of one design, though it was a building, not a character). If they load the image into their 3D application it's also useful to be quite a huge image. There's also the question of low T-Pose vs real T-Pose. I find the latter a pain to draw, but if the character is supposed to get a climbing animation lateron it is recommended. Colour schemes and material information are also quite important, especially if someone else does the texturing.
Note: For side view, I tend to put the arms separately next to the side view body if there is something important on them.
But in short, it's best to simply ask the person what they need and work from there (and if it's an assignment, ask the one giving the assignment – part of getting an art direction briefing is asking as many questions as necessary to prevent miscommunication. At least that's what I learned).
If I only need it for myself, I tend to make either 3/4 / 1/4 with the face separately in front/side/(and 3/4 if I feel like it) and have a separate front view of the whole character with their clothing in layers (without a face).
Since I'm too lazy for whole expression sheets I usually try to capture the personality in the face's front view, so the face itself practically tells me what it looks like making certain expressions (plus, my default style utilizes realistic proportions, so drawing expressions out of context is a pain). Sometimes, I also include other sketches I made of the character, so it's practically growing over time. I also keep early design thumbnails on it or at least inside the psd file.
Short version: It's pretty much communication with an audience (including yourself and whoever needs to work with the design). Include as much as needed to communicate your design. I've seen notes on character sheets, as well as cropped photographs as material information for a texture artist. If a design is more complex, maybe add detailed sketches, even show how things move. I've even seen loose character paintings in action pose and then a super-tight line drawing next to them in low T-pose, telling the viewer everything needed to understand the design.
Oops, didn't intend for it to be so much text, but that's pretty much what I know of character sheets and creating them. But mostly for game characters. And myself.