Do I need Visual Studio Add-in with Qt 5.1.1 for Windows 64-bit (VS 2012, 525 MB)?
Do I need Visual Studio Add-In if I have installed Qt 5.1.1 for Windows 64-bit (VS 2012, 525 MB)?
It depends on which IDE you plan to use.
If you want to use QtCreator (with only VC11 compiler from VS2012) then no, the add-in is not needed.
If you want to use Visual Studio then the add-in is not strictly required, but highly recommended. Otherwise you'll have to do anything Qt related manually (like running moc and uic, adding defines and Qt libs etc.). A lot of boring work that add-in does without your interaction.