When a recipe says to use butter I always use a substitute, usually one of the butter like margarines. Am I making a lot of difference to the end product or does it not really matter?
As Lonnie says, it's a matter of personal preference, plus it depends on what's being made. I would use margarine for pastry and scones, butter for shortbread, croissants or biscuits, olive oil for bread, salad dressings and frying vegetables and groundnut or corn oil for mayonnaise.