I used pennies, then quaters.
When you move around your speakers to repostion, pennies or even quaters may not stick with the spikes, may end up scratching the hardwood floor.
Now I use speaker isolation cones. They are more expensive ($10~20), but they worth every quaters.
I am more interested in how people treat their hardwood floor surround the audio systems. Many times I see rugs (or some sort of office carpets) underneath the whole audio systems or inbetween the listening position and the speakers.
Would it be a good practice to put such rugs to dampen (or prevent reflection?) the sound from the speakers?