PDF standard is a everything including the kitchen sink type solution (so much so that you can not find applications that support ALL features of PDF). You probably have no need for many of the PDF features added in 1.7.
I mean do you really need?
- Improved support for commenting and security
- Add comments to 3D-objects
- More elaborate control over 3D animations
Granted you may need:
Embed default printer settings such as paper selection, the number of copies and scaling.
But I am doubting you actually use this function!
Basically if you do not need, or use, any of the new features of 1.7 then why save as 1.7? It is just waste of effort, at the cost of compatibility. So if you gain nothing, why lose the benefit?
Now there are reasons to save 1.5 and 1.6 though. As 1.5 allows transparency and 1.6 allows embedding open type fonts. This said: There is a benefit of saving the file without transparency when you send a file to a printing agency.