You'd only want the NDK if required. It's easy to detect - you look for the jni subdirectory in your Android project directory and see whether it contains an Android.mk file.
The NDK is used to create a jar file that is then used in the regular Android SDK build, so the NDK build is just a step that comes before the rest of the usual Android build.
In the event that Google changes their doc without notice, here's the info directly from today's NDK Getting Started:
Getting Started with the NDK
Once you've installed the NDK successfully, take a few minutes to read the documentation included in the NDK. You can find the documentation in the <ndk>/docs/ directory. In particular, please read the OVERVIEW.HTML document completely, so that you understand the intent of the NDK and how to use it.
If you used a previous version of the NDK, take a moment to review the list of NDK changes in the CHANGES.HTML document.
Here's the general outline of how you work with the NDK tools:
Place your native sources under <project>/jni/...
Create <project>/jni/Android.mk to describe your native sources to the NDK build system
Optional: Create <project>/jni/Application.mk.
Build your native code by running the 'ndk-build' script from your project's directory. It is located in the top-level NDK directory:
The build tools copy the stripped, shared libraries needed by your application to the proper location in the application's project directory.
Finally, compile your application using the SDK tools in the usual way. The SDK build tools will package the shared libraries in the application's deployable .apk file.
For complete information on all of the steps listed above, please see the documentation included with the NDK package.