Most of the times, when I open an Office document from SharePoint web into the Office client application or Office Online, the file is getting saved straight away.
I find this behavior undesired. Sometimes I just want to open a file to read it without making any changes. So... the 'Modified by' and 'Modified' properties should not be changed. However, as you may understand the AutoSave feature leads to these properties being changed even though no content changes have been applied.

In my opinion this should not happen and therefore should be subject to change. It is just VERY ANNOYING.