It would take a very strong magnet - much stronger than what's in the seam guide - to harm your sewing machine. But the other options are good. On my Bernina, I rely on a 1/4" foot that has a guide on the side, plus moving the needle one click to the right. On my Singer 301, I use
this tape. It's also good for putting on the bottom of your rulers to keep them from slipping.