"But what is so special about invoice forms ? "
Nothing really. I have worked on invoice reports that have been created from scratch as well as fill-in-the-blank types.
The major advantage of pre-printed invoices and forms in general is the use of color!
Generally while the invoices displayed to the screen can hae all kinds of colors in logos, letterhead, etc, the printer used to print the forms is not a color printer due to cost.
So if you have fancy logos consistant with your business (say you were a graphics arts company)you would probably want to see that flair on your invoices as well...hence the use of a pre-printed form.
Usually one printer is allocated to this function (printing invoices) so make sure as SV says that you optimize your report to this printer. Usually, because of slight differences between printers, I don't left or right justify my data to the absolute edge of the box that it will be printed in. I usually move it 1/8" over if possible so variations have less effect. Jim Broadbent