I think Golden Ages are the way to go in the later game. Early on, there are much better uses for GPs and your empire is too small to make the golden age worth the cost of two of them.
Towards the end, though, your production and commerce should be enough that the GA boost to it is more valuable than the couple of turns they'll take off of whatever you're researching and settling them doesn't help out too much.
I usually use around 1500AD as the cutoff date between using GPs for techs/settling and hanging onto them for GAs.