Can I safely uncheck the Optimise Mac Storage check box on the Apple ID - iCloud page?
Yes, it is perfectly safe to uncheck that box.

When you uncheck it, the computer will start downloading your files from your iCloud Drive (if you have any files there, that are not already stored on your computer).

Unchecking usually means that more space will be taken up on your computers disk drive, but that is fully safe and does not make your computer slower or anything like that.

If you decide against it later, you can always check the box again to let the system automatically manage disk space.