>Even after having served his time, there's still a certain stain there

In Western jurisprudence the basis for society enacting punishment on criminals is that the punishment is just. One characteristic of that is it that it is proportionate which means it has an end. Criminals who have done their time don't have "stains" or marks as if they're Cain, they have a right to reintegration, because if they didn't, what exactly was the punishment doing?

So no, it doesn't reflect badly on any software project for employing a person who has payed for their crime. It reflects well on any employer if they treat ex-convicts equally.