If you are buying goods after landing, no, do not declare them.
If you bought the items before landing, and want to transfer them over later, it is in your own interest to declare them as that will make it tax-free, from what I understand. If you ship items later that you did not declare as goods to follow when landing they will charge you duties for importing them.
if one lands with just one suitcase, an in a few months while traveling will get more goods - are they still required to declare?