It depends on how you're playing and on what you're trying to achieve. Plan at the very beginning of the game, usually based on what the land is like around your capital and how many resources there are, how many cities you want your initial expansion phase to consist of.
If this is quite a high number then the opening policy of Liberty is often worth taking and should then be leveraged by hard-building settlers in the capital. If Liberty isn't taken then rush buying settler/s with early gold is certainly viable.
One thing you should be trying to do is get the National College built in the capital either before your first additional city is founded or as soon as possible after your second, or third. This should depend on how many really good city sites are around you and how important you think it is to grab some land based on what your scouts discover about your surroundings.
I almost always use my first 1500 or so gold to rush buy a library in the capital and then to rush buy at least one worker my second or third city, and often I'll rush buy another library to hasten getting the National College underway.
I'd prioritize this over granary/monument.