Is it important to unRoot Android Before Installing Sailfish?

Technically I think that when you initialize the phone when you first start it after installing Sailfish, you are at that point prompted if you want Android Support installed. If you choose that you want, the system.img containing Android will be downloaded at that point from Jolla's server.

But fundamentally you're exactly correct.

@mynameisnotimportant :

Your previous ROM's /system (including the su binary) are not used.

...should be... "Are overwritten".

The system partition which normally hold the "/system" directory (and root) on Android is overwritten by the flash process and is used to hold a recovery image to be used if somebody needs "factory reset".

The data partition which normally hold the user's data is overwritten used to store Sailfish (on Xperia smartphones: using an LVM-based partition scheme to keep the system's root and home separate).

As @teemu said: The android system used for application compatibility is downloaded by the Jolla application store. (It's not available in the flashing procedure, it's only accessible in the store to users with paid license), the compatibility layer doesn't use any pre-existing android on the device (of course, that got overwritten by that point).