Ok! Job done! Here is the solution.

1)Find your category ID you want when starting the point of sale. Here I want to open category AS, id 34.

2) In the file /home/odoo-test/odoo-test/addons/pos_lapagept/static/src/js/widgets_as.js, we replace only the Id 34. All the file is unchanged for the rest.

        set_category : function(category){
            var db = this.pos.db;
            if(!category){

               #this.category = db.get_category_by_id(db.root_category_id);
                this.category = db.get_category_by_id(34);
            }else{
                this.category = category;
            }

3) in the file /home/odoo-test/odoo-test/addons/pos_lapagept/lapagept_templates.xml, we have this :

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <template id="assets_backend" name="point_of_sale assets" inherit_id="point_of_sale.assets_backend">
            <xpath expr="." position="inside">
                <script type="text/javascript" src="/pos_lapagept/static/src/js/widgets_as.js"></script>
            </xpath>
        </template>
    </data>
</openerp>

4) And now, the category AS open automaticaly when we start the point of sale. Great!

Just comment if it miss something. Thanks.