The best way to RIP a file out of InDesign, and print an envelope or awkward in size document — IT IS A MUST to select “custom” to assure that image is placed correctly on plate along gripper edge. Also, by selecting “custom”, the document size will be shown in preview. If crop marks are selected and “custom” is not pulled from menu the press operator will be upset (pissed) as he/she will not be able to align correctly to grippers edge and the registration of a 2+ color job will be off.
RIP away!