If I can manage it, I avoid buying L2 Farming until I'm ready to buy L3 -- in a best case game, I would buy L2 Engineering first, then L2 Science, maybe L2 Exploration, then L2 and L3 Farming, then L3 Science and finally catch up on the rest.

The science techs help getting tech points, so I feel like it's silly to leave them until last, when they don't do me any good!