I never build them, except maybe in a SSC (super-science city). An SSC is a city that makes lots of arrows, where you build Colossus, Cope's etc. These things work well together in the same city to produce lots of beakers -often more than half your total. But I don't even bother with an SSC when I play for conquest.
You can usually get beakers more efficiently by raising your science bar or thru trade. Notice that when you deliver a van and get 100g (for example) you also get 100 beakers added into your science box.
Also, I usually build Marco's ASAP and get a few "free" techs by trading with the AI. After you gain more experience, you will see that the AI can't use the new advances as well as you can, so you don't (usually) have to worry about which techs you give them.
