The golden rule of voluntary excess is to make sure you know what you can afford to pay if you have to make a claim.
The more you agree to pay towards a claim, the less cost there would be for your insurer, so they may reduce your premium accordingly.
But beware - setting an unreasonably high voluntary excess may save you a few pounds on your premium in the short term, but if ever you need to make a claim, you could find yourself with a large bill to settle before your insurer will pay out.