Finally sid deserves its name again. GCC 5 is now the default in unstable and packages for the follow-up transitions are (and can be) uploaded to unstable.
What you should not do:
- upload a library package with an open transition issue [1] without renaming the library package or bumping the soname, just saying "Rebuild with GCC 5", and silently breaking the ABI.
- upload a package which builds a C++ library package, which is built with anything else than g++-5.
- having a vacation and hoping the mess is cleaned up when you come back.

http://thread.gmane.org/gmane.linux.debian.devel.announce/2071