The game works on true line of sight for all models. This means that if a crouching pose cannot see over the hedge then it cannot see or be seen and, thence, cannot shoot or be shot. The same thing is true for a model that is mounted vs one that is on foot. The rider can see over things that the footman cannot. The poses can sometimes lead to problems like that but it is not outside the scope of the line of sight rule. The idea of declaring a model to be in a position other than its modeled poe can be done I suppose but aside from laying down it creates problems with the line of sight rule more than it seems to fix things in my experience.