It's generally safe, but your Mac will have bigger problems than grease on the screen if you put it in bags where it gets pressure that causes the screen to touch the keys. I would avoid those situations or use a thin cloth on the keys if you are worried about grease transfer, grit and surface wear on the glass.
Optically, you sacrifice a lot with any film - even the best 3M quality ones. Unless you need a privacy filter or just feel better with a screen protector, I would recommend taking other measures to counteract your various issues and leave the glass as Apple designed it. Also, keeping a film clean is much harder than keeping glass clean.
Also, if you have wearing of the glass, I have seen Apple cover that under AppleCare - they chose and designed the coatings and surfaces and make tradeoffs between gaps and durability. If you expect a function like retina screen to endure normal use - that retina screen resolution is not functioning properly if the glass has become worn.
I also have seen repairs declined to be covered under warranty (you can always pay for a cosmetic repair if you wish) when it's purely a cosmetic issue - not actually interfering with the proper function of the hardware:
- Anti glare should last reasonably well
- If there are no signs of damage or mis-alignment, gaps in the screen could either be covered or not covered.
Without knowing how you use the computer, it's hard to generalize what's the best tradeoff for your use case. Hopefully discussing your points helps you and others decide what to do.