Al,
Actually it's easy.
As the rivet wheel pretty much keeps the direction by itself, I found that the best way of rolling rivets next to the existing panel lines is to do it freehand.
For all the rest, I make rudimentary pencil marks where the lines should go, then use flexible ruler as a guide. Placing the part on my cutting mat and then using the yellow gridlines as visual guides helps to keep the rivet lines parallel.
Also, whenever I have two symmetrical parts, like wing halves or stabilisers, I work on both halves simultaneously to reduce the risk for differences or omissions.
Everything is easier if the riveting part is done before the wing / fuselage halves are put together.
I was once encouraged by a modeler who rolled all his rivet detail freehand, to good results, with the rationale that any discrepancies (and there were many) would not be noticeable on the finished model. They weren't. The whole process on a Tamya Me 262 took him something like 30 minutes.
