"IF" a firing pin is properly shaped, does not have an overly long projection & moves freely in it's hole, a retraction spring should not really be a "Need". On a gun which originally had them, then they will be a necessity to prevent that over projection. On one which did not I would put pin correction at a much higher priority than an attempt to add them.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra