A boot should fit like a handshake. Ever had someone grab your hand for a handshake and squeeze across the knuckles. Ouch! But if the same person would grab your hand behind the knuckles and squeeze, it wouldn't hurt. That is just how a boot should fit. It should not pinch across the ball of the foot or the toes, but should be comfortably snug in the instep area of your foot. Just like a good handshake.
A new boot may slip some when new, but if it is snug in the instep, that slip will come out, after it is flexed from wearing. It may take a day to two weeks for the slip to stop. How do you know when you buy it if it is slipping too much? If it is so loose in the instep that you can pinch up a quarter-inch or more of leather then it is slipping because it is too loose. It is letting your foot slide forward away from the heel. That is the wrong kind of heel slip.
A good fitting boot does not need to be broken in to make it comfortable, but breaking in a boot will increase the comfort. A new boot should be comfortable from day one and just get more and more comfortable.
Can an improper fitted boot be made comfortable? Sure, see this post on boot stretching.