I currently use a D-40 but looking at a D-90. The D-90 lens is 67mm vs 52 mm on the D-40 lenses I have. All my flters are 52mm. What would be the effect of using step-up and step-down rings on my lenses?
Apparently you have lenses with filter diameters of 52 mm and 67 mm. They remain the same whether you put them on a D40 or D90 camera.
It is OK to use a 67 mm stepped down to the 52 mm on the lens diameter. It is not a good idea to use a 52 mm stepped up to 67 mm since you will have a 7 1/2 mm difference which will create a vignet.
For an explanation for rings see this and click on the help to see how the filter would be on the camera.
hope this helps.