It's generally safe on Macs without an anti glare coating and generally a bad idea if you ever want to go without the cover or care if your anti glare coating gets damaged.
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 and abrasion on the glass.
Optically, you can sacrifice brightness and resolution/quality with some films - even the best quality (3M brand come to mind) 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.
The one benefit of a film is if the adhesive is done well, it can cover up a damaged anti glare coating and also take surface nicks, keyboard rub damage and protect the gas if you Carr your Mac closed in a backpack where it can be pressed or jostled.
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.