I used to have a house rule that PCs would gain a rank in Spot, Listen, and Sense Motive automatically every 3 levels (i.e. a rank of Spot at levels 1, 4, etc., a rank of Listen at levels 2, 5, etc.).
It was not so much whether it was a class or cross classed skill or not, it just made no sense to me at all that high level Adventurers would be blind, deaf, and stupid after surviving hundreds of combat and NPC interactive encounters.
Since it was not a cross class becoming a class skill issue, those classes that are really good at this type of thing benefited nearly as much as those who were not (i.e. it cost them less to max out these skills since they got some ranks for free, hence, they could boost other skills).