Yes, the largest prime number is odd, but if you add two odd numbers together, they become even, same as if you add two even numbers together. Even numbers are easier to equal out that way maybe, 2 and 2 for four, but you could have 2.5 and 2.5 for 5 so that doesn't mean anything. Well, either way, it's really not something anyone really thinks about.
Most definently evens vecause, well, they are even so I can nicely pair them up and plit them etc. I also prefer right to left but thats a different topic.