For me I always left it as Compiler default, after a long struggle with C++11, that was affected, when I tried to use something else… especially when linking to the AAX library. Since then I never had any problem, I can support back to 10.7 easily by setting the deployment target (haven’t tried 10.6, but I think it is old enough to forget about it… that was the time point when there were architecture changes, so that was for me reason enough to not bother about <10.7)
For that topic have a look at: What's the earliest OS people are compiling for?
HTH