What is Stream Saver?

Stream saver is designed to reduce video consumption,  saving you data.if you have unlimited plus or a high data allowance then it may make sense to turn it off. However if you don't want to burn through data fast you should leave it on which still gives you a quality image. 

No matter your choice when on wifi you'll always get the best quality possible based on what your provider allows. 

Whether the it is better to leave on or off is a personal choice. As noted, the purpose is to use less data when streaming video. If you don't notice any difference in the video quality, leave it on.