I pretty much always bulb my GS- I'm gonna be trying to get those techs anyhow, and given that tech growth is (or should be) geometric, even a small early bonus counts or a lot. In G&K, (afaik) it just gives you a multiplier of your tech production, so no reason not to, although I still try to shoot for a spendy tech. I also push all my cities into hyper-science mode (specialists etc, even at the expense of starvation) before doing it, because like I said - its a multiplier of current science production.
The exception is, if I have a city w/ national college & / or observatory early in game (pre-industruial) , I'll plant them. Usually that is also a city with a lot of trading post jungle tiles & a university, maybe some undeveloped banana resources, dedicated almost entirely to science and population growth.