Yes, the position of the airplane on the ground, and wing dihedral can fuss the accuracy of dipsticks. Dipsticks are a 100% certain means of knowing your fuel quantity, provided that the dipstick is accurate. I have never seen an "approved" dipstick. I've seen lots of different dipsticks, and some very good ones, but no one actually taking responsibility for approving one as a "flight instrument" which a fuel quantity indicator in the cockpit is approved to be. The only dipstick I trust, is one I have made or checked myself. This, by leveling the airplane on the ground, and filling a tank from empty, pausing along the way to mark the dipstick at specified quantities, as pumped in.
Similarly I like to personally assure myself of the accuracy of any fuel quantity indicator I rely upon. This includes those of the original airplane, and very certainly, an STC'd aftermarket system. At my client's request, I had the shop install an EDM930 engine monitor in his 182 amphibian - it included fuel quantity indicators. The fuel tanks were newly overhauled, and their capacity confirmed by the fuel tank shop. The fuel quantity transmitters were new, and those specified for the EDM930. I took the plane to the pumps with the avionics tech, to do the initial calibration of the quantity indicators. We calibrated to within a liter. Everything seemed perfect. We also calibrated the "K factor" of the fuel flow, and I did all of the flying to validate the fuel consumption - it was right on.
I returned from on of my test flights in the plane, noting that the fuel quantity at landing was about 3/8ths on both tanks, both by the bar graph, and the numeric indication. The next afternoon, the avionics tech said to me: "You'd cut the fuel pretty close last flight!" Huh? I'd been indicating lots when I landed.... They had filled the tanks that morning, and from that quantity, I had landed with twelve liters usable!!! I go the ugly look from Transport Canada when this was discussed. I should have dipped the tanks to validate the fuel quantity..... I asked the Transport Canada pilot if he was aware of an approved dipstick for the 182, or anywhere in the flight manual where it tells you to dip the tanks to assure quantity - he was unaware of either, which I knew before made the point. And, I pointed out, that it was a completely STC'd fuel indication system, installed and calibrated as specified. Add to that, dipping the tanks of an amphibian, either on wheels, or floating, is not easy, nor particularly safe without a special set of steps or platform. But, the problem remained - the quantity was indicating way wrong, and we could not deliver the plane.
We recalibrated, everything had been right, and was right again. I observed that the fuel flow indication, and thus numeric fuel consumed were right on. So I always knew how much I'd used. What I began to notice that the plane "made fuel" in flight. I could take off with 300 liters, and after 90 minutes of flying, the sum of fuel burned, and numeric fuel remaining in the tanks was nearly 400 liters - something was not working. I phoned JPI, who made the EDM930. I asked if there was an internal check such that the sum of fuel burned, and fuel remaining in the tanks could not increase in flight - crickets on the other end of the line.
We manually recalibrated the fuel quantity to indicate correctly, by "lying" a little as we added fuel. After that, it was pretty close, and erred on the safe side. I still advised the owner to rely more on the fuel flow and burned than indicating. I flew the plane more than 100 hours after, and with the owner, and our manual calibration had been fine. But I was always, cautious, 182 amphibs glide poorly!
When I flew the 310, I learned that the factory fuel indicators for the main tanks were right on, that was reassuring. But, no matter the system, even dipsticks, I self assure before I trust!