On your particular body. I find that sometimes I tend to be constipated and bloated right before my period, like my body insists on retaining all fluids. For me that is definitely NOT the time to do a flush.
But then when my period starts my body is eager to release everything at once. It seems like a great time to "go with the flow," so to speak. When your body is already wanting to cleanse it makes sense to take advantage of that momentum.
My favorite time to flush is when my period coincides with a new moon. Doens't take much to make me happy :)