If your machine is properly calibrated then it doesn't matter which direction it starts or stops. If you have flying optics and auto focus be sure that your beam spot is in the center of your mirrors and lens and that its focused to the tightest point. I have only been engraving for 2 years now but have donated one heck of a lot of time to finding out what works and doesn't. On big projects 48 inch or better I found that doing certain parts of a picture in one file that other parts in another pay off. I can't explain why, just know it works. I know that some machines can do 150 inch per second and if there is NO belt stretch issues due to mass acceleration and deceleration then in a perfect world you can do a huge project in a short time. For me I think the calibration of the unit is most importance to the outcome of the project. I etch at 700 inch per minute, very slow but very accurate. Just my thoughts and my machine starts at the bottom of Y and rasters x from the left to the right.