At this point, the updates to our SDKs only represent functionality updates and bug fixes. You probably won't run into any problems using an older version of the SDK in an integration that is already working. However, I would definitely encourage you to update the SDK if you ever go back to modify your integration.
We do our best to make sure that our SDKs are backwards compatible and will continue to do so for at least as long as they start with 1 as the major version number. Updating your SDK the next time that you work on your integration should only help you avoid running into any bugs that we have addressed and gain you access to newer API methods that simply didn't exist yet at the time that we released version 1.1.8.
