Worthwhile definitely. It means you are paying some of your money a little earlier but if you are billed quarterly and pay monthly at month end you are on average paying six weeks early. I disagree a little with hc4361 who uses a 3% savings rate for comparison purposes. Because you are on average paying 6-8 weeks earlier you 'd need an after tax interest rate of around 20% to make it not worth paying by DD.
I have done so for a long time. Means the money is paid automatically, I don't have to remember to keep it to one side, and I get a discount for paying by DD - and extra Nectar points for reading my own meters and submitting on-line.