I found the answer for my own question....
The exact reasons of choosing Gallic acid as a standard are:
-Partly for historical reasons,
-Soluble in water,
-Recrystallized easily from water,
-Readily dried, and
-Stable in the dry form.
for these reasons Gallic acid is #1, but catechin can also be used as standard in Folin-Ciocalteu assay.
So I think that there isn't a general rule but every standard has different reasons for choosing it.
Hope you find it useful......