There shouldn't be a significant tone difference with a thin steel shim. However, I stick to wooden shims - ebony, rosewood, or maple (left-overs from my purfling stock).
As long as the material isn't soft and spongy (thus absorbing or damping some of the vibration before it gets to the bridge), then there shouldn't be an issue...
The only concern could be with a very thick steel shim, by increasing the mass of the saddle, you will get a slightly different sound. Again, with any standard shim size (up to 1mm), I can't imagine much audible sound difference. Usually, sound difference due to mass won't be heard unless the saddle is changed from bone to brass, for example.
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Ned Milburn
NSDCC Master Artisan
Dartmouth, Nova Scotia
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