When do you research eco upgrades?

I've got my Dark Age build order down pretty well, but one thing I still sometimes struggle with is knowing when research the economic upgrades, especially Wheelbarrow and Hand Cart.

Do you upgrade as they become available in each Age, or is it better to advance first and upgrade later? Wheelbarrow and Hand Cart are especially tricky because you can't research them while advancing (well, you can with Hand Cart if you build another TC, but still).

I can usually come into the Feudal Age with a good amount of resources, but I often wonder if I should just try to skip right to the Castle Age or upgrade and create some villagers to boost my economy while I'm advancing. Thoughts?