This was also my experience. The first prestige I made, I did kind of randomly around level 7 to see what would happen. Every subsequent prestige, I did after maxing out the upgrade tree, and finding myself unable to meet the next level target. Short of the first prestige, I consistently found myself getting less value out of things. This was confusing, since if I just invested in a bunch of prestige upgrades to get more money, and then end up blocked with less money earlier on, something has gone terribly wrong.