Of course numbers should be checked before dating.
I think a reason for not putting it online would be that once you know a number, you may be tempted to put together any possible ones, and would start to figure out who has recessive genes. I think the point is that people shouldn’t know who has recessive genes unless necessary to remove stigma. The point of Dor Yesharim is to help prevent genetic disorders in a way that doesn’t stigmatize carriers.