I decided a few years ago to start tuning to A=440 and trying to get within plus or minus 3 cents. So I did that with quite a few melodicas. I have drifted over time toward A=442 as Daren suggests (or perhaps A=441). I think it’s best to start out by determining the standard to which the melodica is already closest, which in general in my experience is usually about A=443. In deciding the target standard for the particular melodica, I prefer not moving further than I have to from the existing standard. The risky notes to tune are the ones that are extremely sharp (extremely flat is a very bad sign, the reed may be failing).
If I have a reed that is not responding predictably to my scraping, I let it go at 5 cents off. If you tune your melodica today to your specifications and test it a week later, I can just about guarantee that at least two or three reeds will be more than 5 cents off. I think this is more a matter of the difficulty of getting consistent readings than it is of reed instability. I think it will drive you crazy if you keep checking the tuning and striving for perfection.
I agree with Daren about your D-4 reed. You can usually tell by looking whether one reed is more open or more closed than the others. I use a flexible plastic toothpick to gently depress or raise the end of the reed, pulsating, springing it up or down (probably down) a few times.