This will probably sound more rant-like and/or :argue and/or :nono than I intend but anyhow....
The direction doesn't matter (with one exception) and circles aren't bad if you're doing stuff properly.
Nobody says "the PC is bad because it's random orbital"; people like it and it goes in all directions. People like it *because* it goes in all directions. The PC is just a "fast hand that doesn't get tired". Try to be a human PC.
Directions only matter if you're gonna have left-over marring (with one exception I can think of, see below). In that case straight scratches are only visible from certain viewing angles while circular ones are *always* visible (for the same reason).
If you're polishing, your last step will be a mild polish that doesn't leave marring.
If you're waxing, you shouldn't be marring the finish anyhow.
So (other than the exception below) direction doesn't matter unless you're messing up and scratching the car. Don't scratch the car.
Use the direction that's comfortable for you and that seems best for the work you're doing.
Exception to the "do what you want" approach: I often go across/perpendicular to an isolated straight scratch so I don't end up polishing a "trough" by going over it along its length repeatedly. The abrasion from the polish occurs over the entire area you contact, not just the scratch. You want to effect a gentle blending of the levels of the scratch and the surrounding paint. So for a straight, deep, isolated scratch, IMO it's best to *not* just go parallel/up and down the length of it when polishing, go across it (or in circles/etc.) instead.
