On my arm with my cartridge...
- using plastic bolts (or nuts, or washers) provides an insufficiently rigid connection, resulting in looser bass and softened dynamics.
- inserting a metal plate causes resonances/reflections of stray energies emanating from the cartridge, which produces mud at lower frequencies and out-of-phase echoes at higher frequencies.
But YMMV. I only know those things because I've heard them - on my rig. I haven't tried them on your rig and there's no extrapolating from one rig to another for minor tweaks like this.
The only way to find out is to try it and listen (one change at a time, so you know what's causing what). Trying new things helps develop both our setup skills and our listening skills. The experiences of others can help us avoid major errors and give us good ideas, but in the end we have to try for ourselves, listen for ourselves, decide for ourselves.