The spring is already compressed to fit inside the mechbox. The loss of spring power would be do to a phenomena called creep:
http://en.wikipedia.org/wiki/Creep_(deformation)
The spring is already compressed to fit inside the mechbox, being compressed an extra few inches will not increase the rate of creep by a large amount.
The only reason I do the few semi auto shots is to take the stress off the piston teeth.