I shouldn't matter much wich you way you choose. If you can keep the intensity constant that its.
Sometimes it's better to start with abs becasue your "fresh" and can apply yourself more to the task.
there is nothing wrong with doing them at the end of your routine, except, your more tired and may not be able to complete the same workload.
An quick answer is that if your tring to bring about a lagging bodypart, train it first when your fresh.
you can also alternate. One week you do abs first, the next you do them last. It can also be helpfull to do them in between sets of other excersizes.