Hammer follow generally happens when you get your trigger pull too light as either the sear bumps off the hammer by itself or the trigger "bounces" on to the sear from the hammer.
The things that prevent trigger bounce are: Sear spring pressure on sear, sear spring pressure on the disconnector, series 80 plunger spring, and friction/fit between all of these moving parts of the trigger. If any of these are lightened or if there is any tolerance/slack problems, the trigger will be more likely to bounce.
The things that hold the hammer back are the pressure on the sear (sear spring), tension on the hammer (mainspring), and quality of the hammer/sear engagement. Obviously, if you lighten any of these , the hammer is less resistant to following.
With all of the above modifications: polished internals, hammer/sear stoning, heavier recoil spring (which has more forward momentum going back into action and creates a bigger pulse) , and depending on the sear spring - the gun is more likely to trigger bounce. This is more of a concern if you are in the habit of releasing the trigger after some or all of your shots rather than holding it back through the recoil cycle until reset.
If you have reduced your mainspring "too much", thereby lessening the resistance to the "bounce", you may get follow. Notice I didn't say the gun would necessarily fire. In series 80 guns, it is possible for the bounce to cause the hammer to follow but the series 80 levers may not engage the safety enough to unlock the firing pin.
A particular gun may be safe below 17lbs (It could be assumed that a 17lb spring will set to 15-16 lbs after use). But at some point, the gun is going to be unsafe whether it is because the hammer follows, the trigger pull is so light that you bump it and goes off, or that ignition is so inconsistent that you don't know if it is going off or not. It is more likely that inconsistent ignition would occur first with the heavier firing pin spring but the point was more related to cutting hammer hooks and not proper ignition.
As others have already said - I would recommend shooting it a while to determine what you need/want and don't just do someone else's shopping list.
[This message has been edited by James P (edited 05-24-2001).]