The Tennis Ball Roller

An excellent way to apply some self massage to the low back is with a tennis ball roller.  Here's how to make one from two tennis balls, a sock, and a wire tie wrap:

This is great for easing tension in the muscles running along the spine. The spinal processes(the bony bits you can feel) go into the notch between the two balls and keep the roller aligned.  To use it, lean against a wall in a partial squat with the roller between the wall and your back.  Now squat deeper and stand taller to roll over the muscles.  This is especially effective for the low back(lumbar) muscles, which need the pressure without moving the vertebrae underneath.  It will also feel good on the upper(thoracic) spinal muscles, but they don't benefit as much and don't really need relaxing.  In general for most people, the lumbar area needs muscles relaxed but joints steady, the thoracic area needs the joints loosened but muscles strengthened.

Also see Managing Low Back Pain with Exercise if you have more than just tightness.

To address mobility limits in the upper spine, try this.  Lie on the floor on top of the roller, starting at the mid-back.  Please note that this requires good core strength to do safely.  Keep the roller still as you raise one shoulder diagonally up and over toward the opposite hip, with a slight abdominal crunch, about 5 times.  Bend toward the other side, then roll the ball an inch or two closer to your head and repeat.  The point here is to get the spine moving, just a bit, where the tennis balls are pressing.

Worth mentioning is the more familiar "foam rolling" for self massage, using a long cylindrical piece of foam.

My opinion is that, yes, it can be helpful.  It just isn't quite as essential as some folks say, and not always for the reasons assumed.  There may be some benefit from the pressure, but I think the real benefit is from mobilization.  You put your body in unusual positions and move around in a strange ways, and that means bending joints and using muscles in ways that they don't usually get.  The pressure of the roller is just a plus.  If you think it helps, go for it.