Each monster has a "preference". This preference might be a class or a sex or a race (in MM7). So, for example, try fighting vampires in Shadowspire with a mixed party, which has a single cleric/priest in it. You will see EVERY ranged and ALMOST every melee attack of the vampires hit the cleric until he is out. Whichever side you turn to the enemy, cleric will stil be prime target to take hits. Thats the preference in class.

Same way trolls in MM7 target women and priests, so you'd better make your priest male so he doesnt get squashed that often by them.

You can see the preferences in numerous monster tables out there on the websites, and when making parties in MM7 i even considered those so in most situation my party is evenly "liked" by the monsters, so i dont get situation like that in Shadowspire when my cleric goes out first whatever i do (you dont have that freedom of making up a party in MM8 unfortunately)

Thats for certain. Now for the speculations, i have noticed that it sometimes matters where from does the projectile arrive. It seems that when i get shot from the left, my leftmost character is hit more often and vice versa. I dont suppose this is true, but probably.

The only thing certain is class sex and race preference of some monsters.

Also, you might consider that arrows and ranged attacks that are projectiles like arrows fly from the position of your character. So for example if you peep from a corner by slowly strafing to the right and try to shoot the monster, the rightmost character will be the first one who will be able to shoot the monster, meaning his projectile will not be stuck in the wall. So it might be a good idea to have ranged characters at the side.

Although, this might harm you in a narrow tunnels when backpedaling, arrows fired from sides can get stuck in the corners or walls while arrows fired by the central members of your party will hit. So its up to you, i chose not to bother with that

In MM6 and MM7 you cannot dismiss any characters so you can swap them with ctrl clicks indeed.

PS: It even helps beating a minotaur king in MM6 in a temple in Booty Bay where he is prone to get stuck behind a chest, and you can shoot him with leftmost character if you peep from behind the corner. If you go any further he will unstuck, so you have to put your best archer into the leftmost slot to make the process faster.