Not always Barny but the correct way of using ratings is the asessed prices and only betting those that are better than the asessed price BUT putting the money on at the asessed price.
i.e asessed $4 (3/1) price available $6 (5/1).
In this scenario you would bet $33 (the asessed price)
Many proffessionals will only bet if they can get double the asessed price.

Cheers
darky