Doubling-for-less? Insuring-for-less?

I don't know if all casinos allow this, but mine lets me add less than my bet when I double, or insure less than half my bet. Is this ever a good idea? Like, say, doubling-for-less a soft 15 against a three? Or insuring-for-less when the deck is only slightly positive?

Thx in advance.

trufaux