Chris,
Seems we all have the same problem. When printing with sheets, it seems that CS3 with the HP driver seem to get the left and right margins right but never the top/bottom.
In the HP driver I set the Document Size Layout option to "Standard"
Then in the CS3 Print screen, I uncheck the Center Image box and then add 0.25 inches from what ever number appears in the Top box. For example, if after unchecking the Center Image box, the number there is -0.433 inches, I change it to -0.183. It seems to work every time for me. You will have to figure out the metric conversion.
Roll paper drives me crazy too. I think I have just figured out that one. In the HP driver I set the Document Size Layout to "Clip Contents to Margins" and then in the CS3 Print screen, I uncheck the Center Image box and set the top and left margins to -0.25 inches. It seems to work.
Your mileage may vary.
Good luck,
Jim