Class-maps are not applied to interfaces. They only classify/identify traffic.
Class-maps go in a Policy-map, which defines your policy, and then a service policy is applied to the interface that applies your policy to the interface in or out.
As for being effective, in or out, it is totally dependant on what you are trying to accomplish.
The following link shows some Voice class-map type configuration on FR. Hope this helps.
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121limit/121e/121e2/vofvip/vofr2vip.htm
Regards,
Don