John Keegan brings this up this in the Waterloo section of The Face of Battle, if I recall. There was a certain amount ducking and flinching, especially from the noobs, and especially those who where standing next to a row of men that a cannonball just smashed through. The vets would generally laugh at or otherwise shame the nervous ones into standing steady.

Ducking under cannon fire might have been scorned, but it wasn't quite as serious as breaking rank. Maintaining discipline was more important when receiving a charge, but nobody is going to be charging you while their own cannons are pouring fire right into the point of contact. It probably helped that Wellington was pretty good at hiding his formations behind ridges, which limited the amount of artillery fire they had to receive. Nevertheless, I think the British were generally quite respected for their ability to take fire without losing discipline.

According to Keegan, the British Army's secret training method for instilling steadfastness in the face of incoming cannon fire was... lots of booze. Yes, it would probably affect your shooting accuracy, but in the black powder era that was no big deal, since you weren't really aiming at anything too specific, but merely maintaining a rapid rate of volley fire in the general direction of the enemy, or standing steady with bayonets while cavalry tried to break your square.