As a general rule, do not buy any creep upgrdes until you can go straight to Catas.
There are some exceptions where priests early can be useful.
The more of these that hold true, the more the benefit of priests will outway the gold/exp boost to the opponent.
1) If you have high HP characters on your team (Rook, UB), then the healing bonus is siginifcatly greater then usual.
2) If the opponent lacks multiple AoE characters, then it is difficult for them to effectively farm the extra gold, but will still be able to farm the exp. IE: An opponent team of Sedna/Ereb.
3) If you are focusing on pushing where the creeps go, as opposed to going after portals, then you will be able to fight by your creeps and gain a significant advantage. This means, in Cataract, you are pushing the middle, not portals. This is extremely uncommon early in the game.
4) They will help you hold flags. This is a last resort, only in the situation where you are unable to take any flags back.
There are some other things to consider:
If you have priests, then your spawn wave will wipe the opponent spawn wave. This means that usually, if you want to kill creep, you have to venture further into enemy territory to the new equilibrium point. This is risky. Also, waves of 2-3 priests tend to build up before getting killed by towers, giving the opponent easy oportunities for mass-farming.
Some characters benefit in ways more than just extra Exp/Gold. An opponent's Oak is a deadly force in one of your large creep waves. His AoE nuke, combined with Divine Justice, will give him a massive heal every time he kills your creep wave. It is extremely difficult to fight an oak who is nuking your creeps and getting heals from it.