I used to try rat pups as a first meal, with only marginal success, but this year I decided to only offer hopper mice and I've had excellent results (90% +). I try to offer my hatchlings their first meal within a few days of their first shed and, depending on their eating habits or enthusiasm (haha), I usually offer a rat pup by their third meal.
I always say...if what you're doing works for you, then don't mess with it. But, if you're having fewer than 75% of your snakes take their first meal, then I'd say you might want to consider trying what some of the people have had success with here.
Best of luck!
John