The only thing I can think of that would make a test read false immediately following a water change would be a lack of time for diffusion (between the new and existing water) to take place.
The one thing in our glass boxes that immediately assists diffusion is flow, so +1 to refhater.
I would add to refhaters comment something seemingly obvious - I would think two tanks of greatly different sizes with equal flow magnitudes would diffuse in a similar time frame. eg: 50 gallon tank with 500gph flow = 300 gallon tank with 3000gph flow.
Sorry I misunderstood the direction of the thread. That's the difference between "late-night Chalk" and "I've-had-some-sleep-and-coffee Chalk"
I have a 75 gallon with 2100 gph flow (not counting what the power filter does) so my water theoretically turns over 27 times per hour. I personally would feel safe testing within an hour, but I generally wait for the next day, or at least 4-6 hours. Not because I'm worried about getting false readings, but because of time constraints.