There's a problem in that save scumming will make it difficult, if even possible, to create a fair leaderboard.
There are some ways to help prevent this like having the game check a save file's save time against its OS date modified. But it's a very fragile system that can be ruined by antivirus scans. You can screw with the permissions but that involves running the game in admin mode and people (me) don't like files that don't want to be deleted. They can try to hide the save game data by sticking it in some other file that changes file size regularly, but security through obscurity is no security at all.
I believe the best way around that is storing the save files off-site, but that's a lot of money on the developer's part.
In the mean time, I always sleep/hibernate my computers anyway. So I just don't close it.
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