So, your players have refused to pay the Saxons tribute, and the sea wolves are annoyed. You decide (as a GM) that they're going to raid Salisbury. How do you explain that all the manors in Salisbury are raided? Wessex (and Port, later on) are the only Saxon kingdoms close by to send raiding parties, Sussex can make it through Silchester due to their alliance, but how about the rest? Also, how would Wessex make it to the northern parts? Sailing the Avon would mean that they'd be spotted. Sure, they could send hundreds of Saxon warriors running all over Salisbury, but wouldn't they be caught and taken out piece meal if they need to hit every manor? The easy option is to say that only the border manors are raided, which while unfair to those players with manors on the border, is realistic.
So I guess I am interested, how have you solved this in your campaigns? :)