In general, you're best off using the correct size filter. A step up ring is fine, too, but hopefully not such a large step that you can't use the lens hood. In most cases, using a smaller filter than required isn't a good idea, especially with a fast lens.


However, you might be just fine with a step down ring, come to think of it - my 'test' with the 24-105mm was on the 5DII - if you're talking about using lenses with a full frame image circle on your 7D, you've got a reasonable buffer against the vignetting.