I don't think it's all just habit or psychology. If you say 49.72 you need some specific reason for the precision, just like if you said 50.00, but no one (outside of mytho-chartalists) claims that precision so they use even numbers to indicate that it's just a rough guess.
and what about asks/bids or spot rates? I think (baseless beliefs 
) that people are more inclined to type eg 100 than 97There is some resistance to round numbers, and 100 is a big milestone.