I was just reading an article on making half-square and quarter-square triangles, and there was in-depth discussion of the math involved. It said that for the QST, the pieces should actually be 1.20 something instead of 1.25, and that's why people's squares don't come out right. Seriously? That little bit is going to make a difference? I probably have more variance than that just from my level of sewing skill (not expert, that's for sure).
I think cutting larger and trimming down is always better, too.