Personally I would say to avoid any type of ad hoc reward system. It's a slippery slope and can get complicated to track. I'm not sure it really will get you more and better bug reporting than if you thank people sincerely. Just treat them well, give them a little inside info once in awhile to make them feel special, communicate with how you're handling their specific but and if you create any swag, give them a t-shirt or something like that.
As a related note, I have what I call a "Champions" program that I've used at various companies over the years. Doesn't matter if you've got a consumer or enterprise target. Select active customers, ask them if they'd be interested in participating. There's no compensation but an opportunity to help contribute to the success of the company. You give them bits of inside info. Make them feel special. Some swag - t-shirts, whatever. Share with them new features or changes before they're available to the general public...including giving them the opportunity to comment on and shape those features. And so on. Also a resource for testing at times, reporting bugs and related.