Yes, even sitting down you should bend at the waist when shooting out of an elevated stand. If you don't, you will not get the same draw and back tension as on level ground and therefore, you will hit higher. As for feet position, it is OK to have a 90 degree stance or a slightly open stance if you are consistant in your form. If you are shooting an arm eater and there are not a lot of them out there anymore, an open stance will move the string away from your arm more than you realize.