I never delete from the cards. I find it took risky. God forbid you ever accidentally select "Delete All" and lose a whole 3 hours of photos!
I used to do the ratings system but found that just slowed me way down. Now I just start at the beginning, examine each picture and if I want to keep it, I edit it, save it, then move on to the next. It's usually pretty obvious to me if a picture is worth saving or not so there's really no need for the flag system. Plus I'd rather edit too much than not enough for the bride and groom.
I also save EVERY SINGLE IMAGE I have even if it's horrible. I never delete- I just back-up on a portable hard drive. And this has come in handy twice now- three weeks ago I got an email from a former client that one of her guests had passed away and she only got 3 images from the wedding with her in it and she asked me if I would take a look at my tossed out pics to see if there were any of this woman. I found 4 more. They were crappy (the person I was shooting moved out of frame or in one case it was a lighting test shot) but what mattered was getting as many pictures of this woman as possible since these were the last pictures taken of her.
So that's just my 2 cents.