It's maintaining an even pace and your position that counts, not whether you are turning the pedals or not. If you roll better than others - it happens - or if there's a marginal change of pace ahead of you, there's absolutely no reason why you shouldn't coast to make tiny adjustments. It's likely to be better for the guy behind than using your brakes, which will typically cause a more sudden deceleration. Any ignorant person who yelled at me under those circumstances would get an uncompromising response.