Zapatay, don't forget that at some point, you'll want to let the potato plant grow out of the straw and develop lots of top growth for photosynthesis to develop the tubers.
One of my potato towers/"silos" looks like this, and I'm not adding any more mulch. Just keeping it watered:
As you can see, it's starting to flower, which some one has said is when you should stop hilling (it might have been jal... or maybe jbest.... I'm sorry I can't remember... ).