title mostly says it, a good example is that you're playing nasus and you need 200 gold for Iceborn, you have 450 gold, should you get alacrity or should you keep your gold? Generally I think you should save, if you could give me an example which you would/wouldnt would be nice :D
For Nasus in particular, I like to save up until I can get another 10% CDR, and buy then.
In general I like to buy when I know I need to prep for something. I'm a Jungle main personally, and I know exactly when many Jungle Champions can solo the Dragon. As such, I tend to continue farming and save up my gold until just before I know when I can solo the Dragon, and then buy as much as I can before I start. It's also pretty good to buy while you are on or immediately before your power spike, as that extra item can help maximize it.
