If there are things you need anyway, I'd go ahead and buy them and save the ECBs. Otherwise, look for free items until you have coupons.
For instance, like maggiew said, there is a good deal this week on the Old Spice and Gilette body wash. They cost $4 but you'll get $4 in ECB back on your receipt, and you can do it twice. So you will spend $4 (or maybe $8 if you do it twice) out of pocket, but you'll get that same amount back in ECB to use in the future.
So it does take an out of pocket investment at first, but once you get your ECBs rolling, you'll hardly spend anything out of pocket. If I were you, I'd do that deal, and then use those $8 in ECB in the future on products when you'll be getting ECB back in return.