I know for me, no matter how badly off I might be financially, I would never, ever accept any type of loan, salary advance, etc. from either my employer, or my company. I'd turn to friends or other means. I'm a firm believer in not mixing business with my personal life, whether something like this, or dating someone, etc.
I'm reminded of one of my favorite lines from Moonstruck, when Cher's mother is having dinner, and invites a gentleman who just had a drink tossed in his face to join her. Upon finding out he's a professor, and she was one of his students, she looks straight at him, and says "don't sh** where you eat"
That being said, I have at my other job, "bargained" for time off, and volunteered to work other times I might not have, but in that case, its for mutual benefit. For example, I requested a week off this month, but later found out something I thought might be planned during that time, was happening sooner. So I said hey, I really need this weekend off, but will work the sat I had previously requsted. It works for both of us, and since I am flexible, and will "give back" I generally don't have trouble getting time off. Unlike some of my co-workers who will wait until the last minute, when the scheudle is DONE and then say, oh by the way, I need x week off.
And if for some reason, any of my bosses do "allow" me special favors, such as coming in later a couple times, I am grateful, but don't take advantage.