I don not know about distributions other than Gentoo.
On Gentoo Qt is compiled from the sources everytime an update contains a Qt upgrade too.
What i can tell is that Qt might not work reliably if single Qt components are compiled using different gcc versions. So from my experiences to work properly Qt has to be consitend, compiled with the same gcc version, using the same CFLAG and of course the same Qt version.
Different Qt versions can coexist in different slots. From what i know Ubuntu does not support slots, not sure about this.