I don't think I meet the stated requirements of G_F: My 2190C setup is 0.01C res but no better than 0.3C absolute accuracy. That's what I read die temp and wb baseplate temp from btw.
I don't think the raw accuracy numbers are as important as moving your error along in your calcs and being careful to reproduce each test as close as possible. If I find a rare block that I can get a reproducible mount upon and the std deviation for the first 3 or 4 mounts is extremely small then I probably will stop there. If I have trouble getting a good mount I might do more. Be honest! I can't distinguish 0.1C differences in a block's performance. I can't control flowrate batter than 0.05 GPM. I am not confident that these sets of numbers are statistically different. It's ok! Everyone who has tried to run wb tests will understand
I have seen testing very carefully done by people with a minimum of expense (look at Hoot's stuff at overclockers and Cathar's block testing for examples. Probably wrong numbers in absolute terms but very useful and as controlled as they could make them with their stuff). There was a guy at a British website who did some nice testing on flowrate restriction using nothing more than a bucket and a stopwatch
The key is to be honest about the confidence in the results and to repeat tests and become experienced. For all Ben's talk of analytical testing, it is extremely unlikely you'll all come up with a "true" number for a waterblock's performance even if everyone uses exactly the same testing gear. Practice is really the best teacher on this stuff.