I've done it both ways, purchased both licenses and locking tags beforehand and other times waited until I arrived in AK to purchase. The last couple trips I've settled on buying (and printing) my general hunting license at home. I also print out my harvest tickets at home. I wait until I arrive in AK to purchase my locking tags. My reasoning for waiting on the locking tags is in the event something goes catastrophically wrong and derails the hunt I'm not out that big chunk of change.
Hope this helps.