You DO have to scan every body to get your name on that body. I.e., if you scan the star but don't scan each planet you will only be the discoverer of the star even though you ADS scanned the system and can see all the planets.
On a system with a great number of planets and moons, it can take a great deal of time going to each and scanning them. As for getting back and selling your cartography data, you don't have to feel too pressured about taking your time.
I search out Earth Like Worlds and only this last time out (3 months), of the 79 ELW's I found, I only lost 2 to other explorers because they found and turned in their data before me. They were only 1500LY from earth.
Even with hundreds of CMDRs exploring, the odds of somebody finding the system you scanned before you get back is well....astronomical....grin