I've talked about rules many times already on this blog, but another reason that I like them that I haven't talked about is because you can be perceived as kind (and I actually generally am kind!)
My rules are quite tough. For example, I say that I lock the door 10 minutes after class starts, and that if they're even 1 minute late, they won't get their participation point for that day.
But, the way that it plays out in class is that I'll generally give them about 20 minutes before I lock the door. So, if a student comes at 15 minutes, they will be generally suprised and thankful that I'll let them in. They won't be annoyed at me for not giving them their participation point.
And, I'll usually give students their participation point even if they're a couple of minutes later if I haven't officially started my class yet and am doing attendance, or handing out papers or having general chit-chat with a few students. And students realize that they're a couple minutes late and I give it to them and they are surpringly thankful.
So, I think in the end, it's better to be a hard-ass up front and then extend a little grace later instead of having no rules at the start and laying down the law further down the road.