How big a deal is it when you are replacing wheels and the offset is not the same?

Here is what I have: A chevy truck 91' with a 15 to 20 mm ofset on the stock wheel and a toyota with a 15 to 20mm ofset on the stock wheel. I am putting a Hummer H3 wheel on both. The lug pattern matches up good but the hummers offset is 38mm. How big a deal is this going to be? I know it will push the wheel into the wheel well more. Wil it rub?