I don't have a customized listening room, but I've tried pennies under the spikes...it protects the floor, and I didn't notice any problems. I don't know where I heard about it, but it's been mentioned in several hi-fi mags.
Alternatives to speaker spikes?
I have a pair of Paradigm Reference 100's with spikes at the bottom. I'm moving into a newly built apartment with beautiful hardwood floors. What can I do to prevent ruining the floor?