I guess we all have different strategies! My favorite:

L2 Engineering
L2 Science
L3 Science
L2 Exploration (unless I need stews earlier b/c of an event)
L2 Medicine
L2 Culture
L2 & L3 Farming (only if the village needs it, otherwise wait)
L3 Medicine
L3 Engineering
L3 Culture
L3 Exploration

Why? L3 Science costs so much that it's only really effective if you get it very early. If I can't get it very early, I buy it last of all.

And, by waiting on L3 Farming until very late, I'm forced to keep my village small -- which is better for me, because I don't usually have time to watch over a large village before they're self sufficient.

-- Jen