I find I use more gold early on in the game than later on, as they key early on is getting your second city. Later on in the game only use gold for the 5 advisers, however early on I half build times a lot on key buildings like ie the senate.
Couldn't agree more with that.. early on in a world, everyone is on an equal footing and so there's the mad rush for conquest and to see who can take control right from the start. Later on, though, things calm down a bit.. there's more fighting as people have more cities and more resources, but there isn't as much competition to see who can out-build who..
If you have gold, and want to get ahead, the start is the time to use it. The Merchant is the only extended Premium that will really benefit you at the start, as the +30% resource boost gives a small but significant advantage over non-gold players. Administrator is an optional extra, but useful for the extended construction queue if you don't have enough time to constantly keep buildings added to the queue.
Apart from the extended premium options, speeding up construction is incredibly helpful.. if you're on for a few hours, you want to get a lot done in that time and then max the construction queue with slow buildings while you're offline.. but when a building takes 3 hours or more to complete, that's not good. Speed them up a few times (20-60 gold isn't much but makes a big difference) and you can get a lot done in the time it takes a non-gold player to build one building.