It depends on what feels comfortable at the time. I try to stay as comfortable as possible. There's no point on straining the body. That being said, slouching can be rather comfortable, however lately I find that I'm sitting down a lot at work and slouching has been causing plenty-oh-problems. When I'm travelling on a train or subway, I try to keep my back straight. It keeps me awake so I don't miss my stop. Sitting straight can also get tiresome after sitting for 2 hours straight (I get so involved with what's going on at work, I don't even realize how long I've been sitting until I need to look at the time). Right now I'm kind of laying back with a pillow between me and a wall. This is comfortable, but I can already feel my back complaining. I started to have back problems a while back from carrying a tuba in a marching band. I've never been the same since. Ah well.