As Magpie suggests - they probably go through boom & bust periods in the wild, as food availability and weather permits, so it's not a bad idea to simulate that in captivity. Most snakes are probably fed far more frequently in captivity than they ever would in the bush. I feed my youngsters at least once a week, and the adults might sometimes go for 6-7 weeks between (large) meals on some occasions.
For the first year or two it's not a bad idea to feed every 5-6 days to get some size on them, but overfeeding sub-adult and adult snakes is not a good thing. Having said that I wouldn't subscribe to the regime of feeding only after a poo, some snakes are very irregular in that regard (it seems to be an individual thing, as it is in humans!) and it is quite an unnatural approach. It certainly doesn't do them any harm to eat before a defecation, and a snake in the bush will eat where and whenever it can - it won't think to itself "I can't eat that now rat because I'm not going to crap till next week..."
Jamie.