I keep earthing up until it can't be done anymore.

How ever
you can grow spuds without any earthing up infact by just covering the seed potatoes with black plastic sheeting,
as long as the light can't get to the spuds you'll still get a crop,
this method is very handy for people with back problems as its a dig free way of growing,

Don't they produce more potatoes too, when they are earthed up?
I like the idea of saving my back by covering with black plastic .... if I remember I shall earth up as I plant AND cover with plastic next year.
I have been growing potatoes in bags too .... I have used the plastic bags that a large supermarket chain supplied briquettes in. I rolled the sides down to half their height, made drainage slits around the bottom and planted 3 seed potatoes in each. As the plants grow I gradually unroll the sides and top up the soil. Harvesting is easy, tipping and upturning the bag is easier than digging ... and errant potatoes growing in the wrong place the next year is rarer!