The softness of light is a result of two things. The size of light source and the distance to the subject.
A larger light source and/or having it closer to the subject...will make it softer.
So if you use a bag (or Tupperware or whatever) to increase the size of the light source, then yes...the light will be softer. Keep in mind though, that you will also be loosing the light that the diffuser absorbs and also loosing range because you interfere with the beam that is created by the flash head.
Bouncing the light is a very good way to get soft light. When you bounce off of a wall or ceiling...that surface then becomes the light source...and it will probably be a whole lot larger than the flash head. Again, you loose light that is absorbed and you need to be aware of the range/distance because the light must get to the surface and back to the subject. Also, watch for the color of the bounce surface because the light will pick up that color as it bounces.
If I can't bounce, then I will usually shoot direct. I keep a close eye on the image and turn down the FEC if I think it's looking 'over flashed'. Also, I use direct flash when I'm concerned about the amount of light/power than I'll need. As you stop down the aperture, you need more flash power. So when shooting a group or a shot that requires a deep DOF....I don't want to loose power by bouncing or diffusing. This is especially important when shooting outdoors and trying to balance the flash with the ambient.
A lot of the 'diffuser' accesories for external flashes are quite gimmicky. They are good when used indoors, because they allow you to bounce the light off of multiple surfaces and/or send some of the light directly to the subject while bouncing the rest of it. A simple bounce card will do the same thing.
A lot (and I mean A LOT) of photographers seem to think that many of these accessories make the light softer, simply because they are opaque. This isn't really true because they don't really make the light source any bigger or any closer than a bare flash head would.