As already stated, positive labels should be preferred. A typical phenomenon I observe in configuration settings, that you carefully read a label is once, at the first encounter. After that, you skimming for the GUI after sense carriers. So you easily overlook mere negations.
Sometimes antonymes are equivalent and it's hard to say what the positive form is. The two pairs you gave are interesting in this respect.
Disallow Import vs. Allow Import
The word disallow is clearly derived from the word allow by adding a negating prefix.
Hide Settings vs. Show Settings
To hide may be the opposite from show but here it's open which of it is "positive" or primary. Of course, context matters as well, so maybe hide is in "negatively connoted" in GUIs. So maybe a Checkbox should not be used if none of two options is clearly derived from the other, In this case, consider switching to Radiobuttons.