I think allowing open temples of evil deities is an essential ingredient of any grey-toned fantasy setting. That doesn't mean that everyone knows what goes on inside Erythnul's temple, or would be comfortable working with a servant of Vecna - but, socially, these religions would not be persecuted, and many of them would probably have a very strong influence upon society.
I can easily imagine Hextor's clergy sitting at the same ruler's advisory table as Heironeous' clergy, and agreeing on many points with the priests of St Cuthbert.
Similarly, why not take something of a leaf from Planescape's book and have Nerull's temples perform a service similar to that of the Dustmen in Sigil? Burial rites for those whose heirs are too poor to afford it, made all the more attractive by the possibility of willing oneself to the temple to be raised as a zombie. Your family gets a sizeable fee in return - it's possibly the only inheritance the average peasant labourer can really give to his family, and it's not like they ever have to see your corpse toiling in the temple.
Many of the evil deities could moderate their public face to a more neutral model for the sake of the access that social acceptance affords them to the hearts and minds of the common people.
Sure, Pelor might preach that selling one's departed family members to Nerull's temple is giving in to evil, but they can't match Nerull's price - and another essential component of a grey-hued setting is the futility of the efforts that the good-aligned take to combat evil. If Pelor's clergy refuse to stoop to outbidding Nerull's servants for the dead, that just cements their irrelevancy in most people's minds - and ensures their failure.