With weta dollars it doesn't matter. Spend them anytime because the balance of what is left on the item you buy still collects weta dollars, so there is no need to save em. Sideshow on the other hand I try to save em so that I can buy an item close to where I get it for free. For ex if I use say $150 on a $900 Sauron sure I saved but now that's $700 balance that won't accrue points. Weta no matter what or how little you spend you accrue Weta dollars. No need to save em unless you don't shop at all really .
Why wouldn't you save them up to knock as much off an item as you can? If you can take a $250 statue, cut the price in half, and still get Weta Dollars that seems like a smart thing to me.