Originally Posted by repechage

Here is basically how I view angles. Forget what the drawing prescribes, why does one want to know the actual as built angles?
In my book it is to understand the geometry in total, of which angles play a good part in how the bike handles.
That information is in relation to the ground. What the top tube does on a bike that has a "level" top tube is not really part of the equation. It is interesting to note as it might give some indication as to how the mfg practice allowed for variation, but does not mean much when wanting to know the how's and whys of how the bike is perceived by you while riding. So, you better know the top tube length too, and seating position in relation to the bottom bracket.
Noting the actual angles can give insight as to why sister bikes of the same brand don't behave the same, it encourages further investigation of other dimensions, which can play an equal or bigger part. Having the correct measure of the head angle will greatly help in figuring out the actual trail of the steering system, useful in assessing comparison just as much as the head angle.

In short, relying and expecting a level top tube is a lazy way to pull off actual useful dimensions and might well result in errors of assessment. Why waste the time when a more accurate way is easy to obtain that adds useful information in a comprehensive understanding that can be compared between similar fitting steeds?