When creating an AVD, there are three options to select for CPU/ABI:
According to the android developer documentation you should actually create a virtual device for every framework and platform you intend to run the code on, that being said the majority of mobile devices in the market currently run on the ARM architecture.. you should still test your code on a distinct virtual appliance for each level of the framework that your code supports though.
Recommended. Although you might have one or more Android-powered devices on which to test your app, it's unlikely you have a device for every version of Android your app supports. It's a good practice to download system images for all versions of Android your app supports and test your app running on them with the Android emulator.