Filter step-up rings

[Vertigo1]

What is the general consensus on filter step-up rings?

I have a Hoya circular polarising filter for my 17-70 (72mm filter size) and, rather than forking out for another one for my 70-200 F4 (67mm) I was thinking about getting a step-up ring so I can use the filter on both lenses.

Are there any downsides to doing this that I should be aware of? The price differential compared to buying another filter are significant.

[h4rri]

Wouldn't that be a step-down ring? I have a range of these as some kit uses 72 and some 77 but I haven't tried them yet. I would imagine there is a point where it would cause vignetting.

What body do you have? I would imagine on the croppers it would take a lot to sho in the frame.