If the iPhone was a PC with a Windows-styled registry, I'd abstain from using any third party cleaner app. But since its an iPhone, I wouldn't be worried that iCleaner would break the iOS

Otherwise, the surest way to free up space is to get rid of apps you rarely use (or app data such as podcasts, books, etc) - under Settings > General > Usage. Or cleaning up your Camera Roll. Here's a read on the topic:
How to free up iPhone disk space
Major data drainer apps are Photos, Music (or for that matter Spotify), the Podcasts app (if you are a power podcast listener).