If you are going to put fondant on the cake, you must definitely need to crumb coat the cake. This step seals in the crumbs that are natural in any cake and creates a smooth foundation for your fondant.
If time allows it, I like to crumbcoat my cakes the day before – I put it in the fridge overnight and take it out 15-20 minutes before putting the fondant on so there is a little bit of moisture needed to adhere the fondant onto the cake. If I can’t do this the day before, I crumbcoat the cake the day of, but still put it in the fridge for 30 minutes before covering it with fondant.
If I am able to make the first crumbcoating smooth, I don’t always do a second coating – although it all depends on the condition of the cake.