Definitely shop early and don't buy unless the item REALLY helps. Sometimes that extra week is better spent getting more XP from training to ensure your next quest is a victory.
Not sure I agree with the only buy if it really helps part. Once you've spent the week to go to town, why not buy anything and everything you can which is at least marginally helpful? You have no real use for gold until the next time you shop, at which point you can sell back equipment for the full purchase price. To me, that's the big change from regular Descent: no half price resale. So I tend to spend what I've got when I shop, since I'm only shopping rarely.