Also, I'd like to propose the following:
- "Branching <#>" as a shortcut for "the timeline branches off during or after book <#>." For example, a fic labelled Branching-6 (or "AU:B-7"?) would be canon-compliant through Order of the Phoenix, and possibly part of Half-Blood Prince. I got this idea at the Sectus discussion of AUs on Sunday morning (thanks, gatewaygirl et al!).
- "EWE" as an abbreviation for "Epilogue? What Epilogue?".
Some of the best fics I've ever read are AU.