Do you have to rack the gun each time?

In regards to re-racking the gun for each shot, however your gun functions when dry firing without a laser is how it will function with the laser.

See this video for an example in several different styles of firearms:

The laser bullet does not change the mechanics of how your firearm is designed.  However your gun dry fires without the laser is how it will dry fire with the laser.

When dry firing a single action style gun, like a Glock or Sig Sauer, re-racking is necessary in order to reset the trigger. 

If you use a Double Action (DA/SA) type gun, you can keep pulling the trigger without re-racking the gun each time. 

An inexpensive gun like this would be a Taurus G2C. 

However, for actual training, there are benefits to re-racking the slide each time.  When you re-rack, you take your sights off of the target and you are forced to regain your sight alignment for the next shot.  This simulates the same effect recoil would have.  Recoil only affects your ability to get back on target for the next shot - so does re-racking.  When dry-firing a DA/SA that doesn't need to be re-racked,it is very easy to be able to keep hitting the bullseye once you find it.  This doesn't do much to help you develop sight alignment skills.