
U afre with you, the highest woryover infection risks is when the maternal immunity is waning and before the cpups are covered by vaccinations, so I too actually encourage lots of visitors from the word go, and my bitches love all the attention.
I too do warn people who have a long way to come that before 4 weeks they are not showing much in the way of personality traits for them to distinguish (though I find they are all different from almost birth).
I have had people come once and then to pick pup up, and I have had people visit weekly from a few days old, whichever suits them.
The last litter I had was the first where all the new owners were on email, and it was great sharing pictures giving them all almost blow by blow accounts of their progress.