I found out recently that the more civs have a tech, the cheaper it is to research. So it is often a good idea to pick techs with low cost as the others will get cheaper as the computer players get them. The reduced rate also takes place mid-stream so it does not pay to wait.
If all techs are around 40 turns, then a trick I use is to set one researcher specialist in a city, and set the slider to zero. This saves a good bit of gold over having the slider at 10% and the tech still comes in 40 turns.