My grandfather was in the German army in WW2 and he said they called them Fallschirmjäger - paratroopers - for exactly this reason, they would climb tent walls to drop onto beds that had their legs placed in tins of water to try and keep them off.
Ceilings can be tougher for them. One assumes with enough bed bugs, the numbers are such that one of them is going to inevitably hold onto the ceiling long enough to get position. However it's recommended to move beds away from walls because they can easily climb walls and fall from there.
