Fiona N said:
I'd treat sportives like riding a track with beginners or like mountain biking - falls are more likely to be caused by other cyclists getting excited on the day and/or be at slow speed (on the uphills as people get tired) than by collisions with cars - thus helmet wearing might save you some skin. Plus if it's sunny, they do protect your head from sunburn and keep some of the sun out of your eyes
Rubbish... have you ever worn a cycling cap ? It gives much better sun protection and shade. Modern helmets have so many holes in them I've come back with my head looking like a chess board.
And the peak on a proper cycling cap sits at the perfect angle to see the road but not the sun.
That aside, to answer the OP, I would say that
subject to the rules of the event, wear what you would normally wear. If you don't normally wear a helmet then having one on is more likely to make you feel uncomfortable or irritated, physically or mentally. I find them horrible and sweaty and very hot, and hate every minute I have to wear one when I do.
But sadly a sportive (unlike an audax) will probably compel you to wear one - they don't do anything for your head but they are very good at covering arses.