I see Leia keeping the name "Skywalker" as riskier simply because Leia was adopted by a rather high profile family. Bail's a galactic Senator, after all, and his wife is Queen of a planet. Also, there's the fact that I can imagine that Padmé and Bail's friendship would have been much more widely known. We see Bail go into Padmé's pod once he returns to Coruscant, after all. There might be some (such as Palpatine) who could make the connections. For the Larses, though, the only ones we ever see who visit them are Anakin and Padmé and Padmé is now dead. The Jedi Council likely knew about the family since Anakin would have had to explain why he broke their orders, but it doesn't seem like common knowledge. Thus, Anakin would be the only that Obi-Wan would have to worry about finding out the truth. And Tatooine is far away from the center of the Empire. I think Luke's risk is therefore substantially lower than Leia's. Also, just a minor point, but Leia was actually adopted into the family, so I think it only makes sense for her to take Bail Organa's last name. Luke was raised by the Larses, but they wanted him to know that he was just their nephew. That fact might have played a role. I was simply suggesting that since Jedi aren't allowed to have children, Owen Lars might be a bit suspicious about why the Empire would be hunting Anakin's child in the first place, if such situation exists. What I mean is I think Owen Lars would have liked an explanation as to why the Empire knew about Luke in the first place. I think he'd have realized that, unless there's someone on Coruscant who would have an interest in Luke, there's really no reason for anyone to be looking for him since Jedi weren't believed to have children. If that makes sense. But I imagine that Owen might also have been suspicious of Obi-Wan's motives if he tried to get him to change Luke's name and not done so for that reason as well.