If you are playing conquest, I would focus a significant science effort on my military technology - chivalry, gunpowder, cavalry - and nothing on the non-military science (the great library will look after those)
After you have researched cavalry, I would turn science way down, but not until then. The reason is that I like to wage war with superior military technology. If you are the first to obtain cavalry you will have a huge military advantage, which you can exploit by raising lots of money to field a big army.