I suspect the reason for the SR20 fuel quirk lies behind the tank shape coupled with the wing dihedral, since the bottom of the filler neck (one very nice machined component) is a little way below the top of the tank. It is however a noticeable effect and can catch people out - often the more experienced people who attempt to use sense rather than blind acceptance
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The DA42 (and G1000 equipped DA40) has 2 methods of assessing the fuel on board. The easiest to use is the totaliser but as you say this relies on understanding the system and using it correctly. A particular hazard in the DA42 with long range tanks in the nacelles which have no extra gauges and have to be filled to the absolute brim to guarantee 76+ USG on board. The main tanks have capacitance gauges which are in general very accurate but don't take full account of the tank shape. From experience they can indicate slightly more than the totaliser when decreasing past 20USG/side but then become progressively more accurate again. Fuel reserves are especially important in diesel aircraft because any air locks in the system can only be cleared on the ground so any unbalanced flight or manoeuvring with low fuel can be rather embarrassing! Quite hard to explain this to a more experienced colleague when he handed a DA40 over to me with 4USG left (fuel burn 5.5USG at 70% power) and no apparent understanding of the issue...