In the latest SDK (6.1.4), MACOSX_DEPLOYMENT_TARGET is defined as 10.6

I discovered one of my freshly compiled external doesn't work under X.5, so I tried to recompile it with MACOSX_DEPLOYMENT_TARGET = 10.5 and it seems to work fine (X.5 and X.9, 32 and 64 bits). Is there any risk to do that? If not then why not using 10.5 as default?