just checked, its the same thing. wont make anything faster but is nicer looking:
In [9]: User.objects.all().values('id').order_by('-pk')[0]
DEBUG util (0.000) SELECT `auth_user`.`id` FROM `auth_user` ORDER BY `auth_user`.`id` DESC LIMIT 1; args=()

In [10]: User.objects.values('id').latest('pk')
DEBUG util (0.000) SELECT `auth_user`.`id` FROM `auth_user` ORDER BY `auth_user`.`id` DESC LIMIT 1; args=()