The go to desktop when I sign in feature on Windows 8.1 makes use of the OS setting for that. If you are being taken to the Windows 8 menu on sign in then it would be some sort of OS issue rather than Start8 itself as it only does the heavy lifting for that on Windows 8.0. You should be able to confirm the setting is set correctly by right clicking on the taskbar and selecting properties. It should be ticked on the option to go to the desktop.
The setting unfortunately is tied to another setting in the OS which is what happens when a Metro app is closed. Microsoft opted to make a change in this between the beta and the RTM and as of right now having the setting set to take you to the desktop will make closing a metro app also take you to the desktop. We are investigating if we can provide additional control over this in future updates.
