Your rear brakes should be able to lock up and slide the rear end but should not drag upon release. A drum front brake is difficult to lock up on dry pavement, disk brakes are a different story. Your front brakes provide the most stopping power and you should adjust them so that they can be locked up also on dry pavement but shouldn't drag, if they will lock up at all. Learning to use and control them together depending on the road surface is something you'll have to master, don't just rely on your rear brakes all the time. In an emergency situation you'll regret it.