When it comes to typographic design, it can be dangerous to adhere to rules of thumb. Sometimes (actually most of the time) sans-serif fonts work, sometimes they don't. It would depend on tradition, trend, and overall feeling that the fonts project.
If the journal does not specify. I would usually favor sans-serif. The reason is that unlike my texts, I am not always sure how much the editorial team may size down my illustration. Sans-serif has a pretty good property that they are quite resistant to shrinking, and can still be legible at relatively small size.
In the mean time, if the publisher uses any software to smooth out the edge of the fonts after resizing (e.g. through aliasing,) serif fonts can sometimes appear broken at their thinner strokes.
There are, however, some illustrations that just don't look right with sans-serif. For instance, line labels and angle labels of trigonometry problem sets and formula like this one are much nicer with bold and/or italicized serif fonts, monotone ink-drawn anatomical charts (like this one) will just look very odd if we put on sans-serif labels. This timeline describing Shakespeare's Life may look ridiculous if sans-serif fonts are used.
In those difficult situations, look for serif fonts that are beefier or with more uniform stroke width. As they can likely withstand shrinking and aliasing. In addition, look for fonts that are slightly wider, and have a good "x-height" (literally height of the font "x"). Some possible candidates are Caslon, Baskerville, Garamond, and Palatino. Avoid cursive fonts, or fonts with some very thin lines like Times New Roman. A more in-depth discussion on squint-free fonts can be found in this blog page and this thread on SE UX.