Cheap Special Effects Explosion
Recently in the news, the above video was broadcast showing an exploding house via a police dashcam. You never actually see the house explode (which takes place off camera), but the results of the explosion.
This is a great lesson on how to show a spectacular event which is typically out of your production budget. Notice that all that really happens (or that is pictured) is that there is a bright flash of light, the camera tilts, and debris flies in from camera left.
This could easily be faked by starting with a normal shot, then tilting the camera at an angle (no action should be taking place so the jump cut will work). When ready to portray the blast, have a ton of people off camera throwing trash into the frame (preferably up, so it will fall into it). Add smoke and a big fan, and you could pretty much replicate the above clip.
In post, add the flash and a loud sound effect. The featured video was actually silent, with all sounds being added by me, found on the web.
If you can really blow something up, I say do it (with all safety measures in place, of course). If you can't, there are creative alternatives that are the next best thing, and will save you a ton of money that could be better put into your movie elsewhere.