Do most devices support side-loading? I've googled about application side-loading (i.e., installing an application on an Android device without having to go through an app store), and most instructions mention that the device might need to be rooted to do that.

I am looking for an Android tablet to host a custom-built application, but I don't want to root the device (for reasons of warranty, stability and support). Do most devices officially support sideloading of applications? I've tried googling but only found results related to particular devices.
Usually sideloading is not prevented by the manufacturer, but by the operator. All the tablets I've tried had no operator customizations and supported sideloading. Of course, this depends on your country and operator.