It's pretty much freehand, but you can add a fence to ensure a consistent distance from the edge each time for some processes. For example, a fence is handy for flanging (Z bends) on the edge of the sheet.
Funny you should ask the freehand question because I don't have an artistic bone in my body and have stressed over things that aren't perfect my whole life. Oddly, I find bead rolling relaxing because even things that aren't perfect still look great (at least to me). It's taught me a lot about appreciating things that are unique and imperfect, but great to look at. I used to trace transfer a lot of patterns onto metal with acetone, then bead roll them. I got away from it for a long time because it is a two man job with a manual bead roller, but is so relaxing to do freehand by yourself with a power roller. I bought a big Baldor buffer on sale about ten years ago for this stuff; it's amazing how polished a rolled sheet looks after a few mins on a powerful buffer.
You can go full squish if your motor is powerful enough, but you will also marr your work that way—especially if you are using steel dies instead of Nylatron or some softer material. Some dies will even cut through sheet if you go too deep at once. Going too fast or deep leaves no margin of error—which I hate. If you ease into it you can adjust as you go. Once your marker lines get erased in the end it's amazing how small errors are difficult to see. You may think you went way off a line, but after erasing your Sharpie lines, you often can't tell because it all looks so good. Also, by doing a slow pass without much pressure you make a path that the dies follow better each time. Good machines have variable speed petals with speed limiting pots, so once the path is established you can go faster. The trick is to count the number of rotations on the handle. That way you roll the same depth every time. I go very slow just because it's amazing how quickly you can stretch the sheet if you get carried away.
For artwork people often use an Art Roll die (very narrow) with a grooved die of a favourite size on the bottom (affects the sharpness of the groove). You do light passes and then switch to step dies to deepen the grooves and provide definition. By using softer material or even skateboard wheels (common trick) for your bottom dies, your beads transition into one another like flames—instead of crushing any previous beads you cross over. It's a true art for those who master it and great fun for people like me that just get "competent" at it. But I'm told every power roller is like a sewing machine in that each one has its own personality.
I'm looking forward to getting back into this side of metalworking, and doing some more patina work as well.
