The Space Bar is both Play and Stop while in Source/Record Editing Mode. In Trim Mode, after completing a trim, just hit the space bar and it will loop play for easy review of your edit.

  
For those on really old versions of Avid Media Composer:
Map the SOLID PLAY button from the Command Palette (in really, really old versions of Avid Media Composer you have to move the Play that is on the "5" key) to your space bar. Now the bar becomes not just Stop, but Play as well while in Composer Mode. Also, now when you are in Trim Mode and have finished a trim, just hit the space bar and it will loop play.