There are three things you need to consider when buying boots: instep, ball and heel.
INSTEP: Unlike shoes with laces, a boot has only the instep to hold it securely to the foot. Hence, proper fit in the instep is of utmost importance. The fit should be snug, not tight or loose. The snugness is governed by the instep fit and the width of the throat. If the instep is too loose, the boot will slip excessively in the heel. To remedy this, you may need a more narrow width to shorten the circumference of the throat. This will reduce slippage without cutting down on the length of the boot. Slight slippage is necessary to obtain a proper fit.
BALL: When you walk or run, you bend your foot at its widest part. This is called the ball. Same with boots. The ball of the foot should rest on the ball of the boot. If the boot is too short, the ball of the foot will sit too far forward and force the toes into the toe box.
HEEL: A new pair of boots will slip slightly in the heel because there is nothing to prevent the heel of the foot from riding up slightly. When the boot is new, the sole is stiff. As you wear the boot the sole is “flexed”. With time, most of the slippage will disappear.
Ultimately, the decision on a proper fitting boot rests with the wearer.