If you are concerned about reselling them, then removing them from the cards will reduce their value. If you aren't concerned about reselling them, and you want to enjoy them loose and display them that way, then opening them adds to their value.
Yes that is true, BUT some Cars are Worth more when they are removed from the Card/Blister, Mike is right, and if You want to find out the Value for some Cars, go to this Link: http://www.southtexasdiecast.com : hope this helps! :wave:
Of course taking out the Hotwheels from there card is decreasing their value.... First of all Dust gets on them and Second of all the person buying it dosn't know whats been done to it outside the box.