The loss of head pressure from tapping high is minimal over the range normally used. If you tapped on a ladder, the head pressure might be noticeably less -- otherwise not really. That small amount of loss is probably offset by the fact that sugar content goes up as you move higher on the trunk (closer to the source of sugars -- the leaves). So overall it's mostly a wash, unless again you're tapping on a ladder. If you do that, it's presumably for a different reason than just to tap higher up.
Tapping is normally done from the height of the lateral line upwards to where it is comfortable to reach. Tapping well over your head is problematic for a couple of reasons. First, it is harder to do and far more tiring (heart has to work hard to pump blood up to your muscles when working above your head). Secondly, it is very difficult to make a good taphole when you're reaching high.
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This is because the movement of the drill in-out when tapping over your head is hard to accomplish without putting a little arc on it. That up/down movement is what creates oval tapholes or drill bit scoring along the top edge of the taphole.
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Not such a big deal with gravity sap collection, but under vacuum, and particularly under high vacuum, that is a point where the taphole may leak.
The better approach is to examine the entire tapping band to find the appropriate place to put a taphole (another whole thread), make sure the dropline can reach that spot, take a stable stance and brace your elbow on the tree, place the drill on the chosen spot, then drill in/out in one quick motion with NO sideways or up/down movement imparted on the drill. If you still have a hard time making good tapholes, consider using an Accu-Tapper.
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As already mentioned, snow height is an important factor. You do NOT want to be shoveling your lateral lines (or mainlines) out of the snow during the season.
Two other considerations on taphole height. If you're using very good sanitation practices and have very good vacuum, tapping below the lateral is a good option, particularly for trees that have been heavily tapped or are growing slowly (big, old trees). Secondly, for people tapping with buckets, they'll often place the taphole at a convenient height to hang/retrieve a bucket, which falls within a pretty narrow height band. At the same time, bucket tappers tend to approach the tree from the same side all the time (again, due to convenience in picking up the buckets when collecting). This can result in cluster tapping. So even with buckets, be sure you're spreading the taphole around, both up and down and all around the stem.