Some good points are being brought up here. I will submit though that there is a lot more drill bit wandering going on than people realize at time. Like Griz said, if your blank is not drilled spot on, then the squaring using the mitre guage on a sander will not work well, as it will square to the blanks outer edges. This can also happen, though with a barrel trimmer. If the blank is still in the vise (Or drilling jig) which holds the blank based on it's outer edges and the barrel trimmer is square with the deck and there was any bit wander during drilling, you might also have an issue with surfacing. Using the sanding jig, allows the blank to be trimmed and leveled to the barrel as the blank is held in place by the barrel.
Also, depending on what material is being used, you will see more or less bit wander. For example while drilling aluminum, I have seen as much as 3/16" difference from entry to exit, even though drilled using a solid drilling jig. Generally, the bit will follow the path of least resistance. If you are using a burl, which has many varying densities, than you might see more wanering too.
This amount is likely to be minimal and really not a big issue on many kits. But sometime, when building a big honker like the "Robusto", a little issues becomes very noticeable when assembly comes around.
Cheers,
Here is a small depiction of squaring and drilling issues made reference to: