This is similar to how I've always done mental addition or subtraction, and "discovered" the method independently, like your DS. My algorithm is a little different, but it's the same principle.

Personally, I find it highly advantageous to have a simple algorithm that works in any and all cases, rather than using the algorithm in some cases, and memorizing "math facts" (don't get me started on how I loathe that term) for others. It greatly enhances speed and accuracy.