How soon to earth up?

My spuds are racing out of the ground... They looked so much bigger this afternoon than they did when I left them yesterday!

Do they have to be a certain height above ground before I start to earth them up. There are still some spuds in some rows that are just having a peek at the world where as others are waving merrily!  

Should I earth up to where the leaves start or just leave the growing tips showing?

These are some of the more advanced ones...