The current behaviour of slime-indentation is to indent them like MULTIPLE-VALUE-BIND.
(with-slots (x y z) object (list x y z))
(with-accessors (x y z) object (list x y z))
However, the Hyperspec and a number of other books seem to suggest that the instance argument lines up with the parameters.
(with-slots (x y z) object (list x y z))
(with-accessors (x y z) object (list x y z))
I've attached a patch that will format these forms like the second example.