Reference the Microsoft.Office.Interop.Excel and set Embed Interop Type to true?
Will doing so ensure my app is backward compatible? (As long as I use safe methods that work on all versions.)
Or do I have to use something like late binding and all in order for it to be compatible against multiple versions?
This link indicates that you get some amount of version independence:
I can't give you a definitive answer though as my brief experience with Office Interop was limited to 2007. It all depends on if Microsoft is changing the interface ID's for the Office interop for each version of Office.
But as long as you keep to methods you know existed in previous versions I think you'll be fine.