Scars for a tummy tuck will end up roughly midway between the upper incision (usually just above the umbilicus) and the lower incision. Sometimes if the upper abdomen releases more than anticipated, the scar will be a bit lower, for example if the patient has had a lot of scarring from a prior liposuction procedure, or has a great deal of upper abdominal looseness. But generally, the incision ends up slightly higher than midway between the umbilicus and the lower incision.
So why wouldn't every doctor just make a really low incision, therefore ending up with an ultimately lower final scar? Because it is not always possible to take out everything between the very low scar and the umbilicus, especially in fit women who do not have have a lot of loose skin. A highly reputable doctor will do everything possible to get a low scar. A lazy or unqualified one may not; I have seen many cases where doctors do not do a sufficient dissection on the upper flap, and therefore the downward mobility is limited, raising the ultimate scar. A wise doctor will also not close things too tightly: this will lead to a wide, unsigtly scar.
In fact, ironically, fit patients end up with higher incisions when they have a conventional tummy tuck!
Location of incisions is important, so doctors and patients should discuss frankly where the incisions will end up. Scar position generally does not change, but scars usually lighten considerably, especially with laser therapy, creams, etc.
Now, there are many in-between short scar procedures that can be used that are neither mini tucks nor full tummy tucks that can allow lower placement of the incisions but still enable full tightening of the fascia.