Lots of heel striking punishes the lower body with repeated forceful impacts, whereas striking first with the forefoot reduces that jarring impact. For example, a walker or distance runner is apt to heel strike, but a sprinter will push off with the ball of the foot - which may depend on speed as well. Some shoes are designed to cushion the heel more for this reason, and others are more "minimal". I've transitioned from heel striking to more minimal footwear (which may have kept me from having some runner's injuries like plantar faciitis and possibly shin splints - but that's another topic).
The "ideal" is what works for you, but walkers heel strike first, since walking is slower; therefore the force from heel striking is minimized and easily absorbed by the body. Runners are the ones who have more reason to be concerned about heel striking. I've read that the force from heel striking while running can be up to 6 times body weight.
When walking, I don't worry about it. But when running, I generally try to plant more horizontally to the ground to minimize hard heel impacts. The height of the shoe heel (in comparison to the sole height of the forefoot) is related to how your feet land on the ground. A shoe heel height of about 12-26 mm is normal for most athletic shoes, and 0-12 mm is considered a minimalist style shoe. Shoe manufacturers sometimes list this 'heel drop' measurement in their product specifications/details. Google "heel drop" or "heel lift" for more info. It's more difficult to heel strike hard in a minimalist shoe, and conversely, more difficult to forefoot strike in a shoe with a large heel lift.
Hope this helps some!
