Is it a requirement to use the "Daydream technical preview" version of Unity with Google VR SDK, even if I am targeting Google Cardboard? I don't think this is clear from the documentation, and I don't want a development environment which is Daydream-specific.
Currently, I have installed Unity 5.5.1f1 and Google VR SDK 1.20. However, in player settings, I do not get any of the Cardboard/Daydream options shown in this screenshot:
https://developers.google.com/vr/unity/get-started#prerequisites
Is it a requirement to use the "Daydream technical preview" version of Unity with Google VR SDK, even if I am targeting Google Cardboard?
No. If you are building for Daydream then you need the Daydream technical preview. Note that there are some sample scenes from the Google VR SDK plugin that are exclusively meant for Daydream. One such example is the VideoDemo. These scenes have Daydream scripts that won't compile on a normal Unity version. If you use the normal Unity version, you will get compile-time errors if you load one of these scenes.
Daydream technical preview is only used to experiment building for Daydream at this moment. It will be added to the normal Unity version when it is ready and with most bug fixed.
In fact, Daydream support is now added to Unity 5.6 beta. You can get that here. I suggest you start developing with Unity 5.6 beta in order to prevent mistakenly using scripts are made for Unity 5.6 beta.