While you donít have to ďhaveĒ a spring loaded firing pin itís a good idea to install one for insurance purposes. Would it stop slam fires 100%? No but it greatly reduces the chance. Iíve not heard of a slam fire with the spring loaded fp. Although I have experienced a slam fire without one. Just food for thought.
(Ha while I was typing this Boris replied, I agree with most everything he says, wasn't even thinking of sear/trigger group issues, but that is always a possibility too)
I would say you have to pick your poison based on your situation. Slamfires with free floating firing pins are most certainly common enough to warrant paying particular care to ensuring that the FP channel is clean, the FP is straight, the FP tip is in good shape, and the tapered fit between the FP and bolt doesn't induce the FP to stick forward. While keeping a clean firearm in good operating condition is not a 100% solution either, it does go a long way to keeping slamfire chances down pretty low.
Free floating FPs can impart a pretty healthy dent in the next round's primer upon chambering and spring loaded FPs definitely help to prevent this. Russian and combloc ammo with their typically hard Berdan primers is not nearly as susceptible to unintentional discharge under these conditions as domestic brass with boxer primers.
The downside is that Russian and combloc ammo sometimes has a pierced/blown primer issue where the pressure profile of a fired round rises above some threshold that causes the primer to fail and allows hot gasses to flow into the FP channel of the bolt. I've seen instances where when that occurs with a FP spring installed, the FP spring melts and causes the FP to stick forward...causing a slamfire that will not resolve itself until the magazine is empty, the bolt can be pulled & the bolt torn down, and the damaged hardware replaced. Always a good idea to regularly check for pierced/blown out primers when firing Russian & combloc ammo.
I would say that the most important thing is to simply become comfortable your SKS: understand how it operates, learn how it shoots in different conditions with different ammunition. Observe how it runs and make note of things that just don't seem right. Keep it well maintained and clean. Think about what you would do *if* the next round you fire is followed by a slamfire of N rounds, as that is *always* a possibility irregardless of how many precautions you take with your SKS (or FN49, or M1 Garand, or ...) Become comfortable with using it and the slamfire issue isn't something you should have to worry about any more than you would any other thing related to general firearm safety.