Are they really necessary? There's at least five tags like this.

avoidable_anatomy_mistake
avoiding_drawing_faces
avoiding_drawing_feet
avoiding_drawing_hands
avoiding_having_to_draw_hands

Is it safe to nix 'em or are they actually useful to some people?

Nixing tags, unless they are inaccurate, is usually a bad idea and in bad form.

Those tags are worth keeping.

They'd be useful for beginning artists, I'd imagine. It helps to learn the tricks of the trade, which involve learning what to avoid.

TheListener said:
...which involve learning what to avoid.

Avoid drawing such pictures, huh? If someone tagged picture with avoid_* tag, that means it's absence was spotted and avoidance failed :)