Nope - I use the CR as a rough guideline when designing.
If I don't already know exactly what critter I want in a given place, I will sometimes scan through the lists by CR, looking around roughly where i want the callenge to be. When I do know what critter I want to use, I will start by checking the CR to make sure my choice isn't completely off base.
In either case, though, I will also consider the critter in specific detail, rather than rely on CR to tell me everything.
If I don't already know exactly what critter I want in a given place, I will sometimes scan through the lists by CR, looking around roughly where i want the callenge to be. When I do know what critter I want to use, I will start by checking the CR to make sure my choice isn't completely off base.
In either case, though, I will also consider the critter in specific detail, rather than rely on CR to tell me everything.