Cables are done on RS rows (odd # rows). You don't want to do a cable on the WS rows because it would never been seen. I assume when you say "purl side" you mean the Wrong Side.
You can do cables on any Right Side row that you want to. I do cables on every RS row all the time. It depends on the design you want to create. There are thousands of possibilities. You definitely don't want to skip an odd # of rows.