If you want to stick with a single render but not waste time "squishing" each clip individually, you can select all events on the timeline (Ctrl-A), then Ctrl-drag the end of the last clip back to the left to speed up the entire project uniformly in one shot.

The only reason you might consider a double render is if you're going to be making many different output formats (.wmv, .mpg, .rm, .avi ... etc.) in which case i would still do the above procedure and render to a new DV file, then create the other renders from this new file. This way subsequent renders only need to encode to the output format rather than process the edits, transitions, speed change, etc. and you'll save a lot of time.