I can't see a group of people as intelligent as the Primarchs not being ready to take advantage of physical appearance. Just look at today's 'stars'; how many of them have plastic surgery from an early age, change fashions like we breathe and basically use their look (and looks) as a tool to get ahead? All of them, some are just more subtle about it than others.
Why would people do this and go through so much hassle if it didn't bring an advantage? How someone looks goes a long way to making those who see them think in a certain way, even before we hear them speak or see them act. Someone who looks 'beautiful' will be given a hell of a lot more leeway than someone who is 'ugly'. So Sanguinius will be allowed to act more freely, but Russ, with his barbarian look, will be allowed to act more barbarian-y before anyone says anything or expects any different.
Looks are a huge part of psychology and the Primarchs are clever enough to use this to their advantage.