Re: when to block feed?
I would try nursing in a reclined position before getting too aggressive with block feeding. It sounds like this maybe a fast letdown issue. I would lean back on a few pillows, put my baby on my stomach or chest, and let him latch. Gravity helps to slow the flow. It can take a lot of tweaking (lean back a little less, a little more, put the baby in different placements on your stomach or chest, etc.) before you find something that works well for you. I had pretty insane letdown and nursing reclined saved my nursing relationship. Some women find side lying nursing helps too.
Nursing very frequently helps with overactive letdown and oversupply too. Your baby may refuse to nurse more frequently, but it certainly doesn't hurt to offer (especially once you find a position that she is more comfortable with).
You can try block feeding, but if you aren't having any other problems associated with oversupply then I would start small with it (same breast every two feedings) and give it a week to see what happens.
K. Sophia - Mama to my little lactivore, the amazing Mr. X (11/10).