I think it's entirely up to you.
You probably know someone, or at least know someone who knows someone, who's been struck by lightning and survived. It's not really THAT uncommon - nearly 1 in 10,000 people are struck every year in the developed world. Many have lasting consequences to memory and/or personality; some have chronic pain.
You'll probably also find that everyone who responds to you, myself included, has ridden and/or run through heavy lightning and not been struck. Except that statistically, there are almost certainly people on this board who *have* been struck by lightning. I would honestly consider the sudden heavy rain that accompanies thunderstorms to be a much greater risk. You can take precautions by avoiding roads that are prone to flash flooding, choosing your lane position to stay out of deeper water, and compensating for the loss of traction and braking power, but there's only so much you can do to protect yourself against drivers of vehicles who aren't taking the same precautions, especially considering the reduced visibility and the reduced available lane surface.