I would argue that having a 'don't ask but encourage' policy with shoes is not really any more polite than having a shoes-off rule.
Sometimes it is good for people to know where they stand.
It did occur to me that having a 'don't ask' policy might cause resentment and division amongst guests at party.
The people who take their shoes off may feel superior to those who have kept their shoes on. Even worse they may feel resentful of those people who have kept their shoes on.
A bigger problem is the embarrassment caused to guests who have kept their shoes on when they realise that shoes-off is preferred. Discovering that shoes-off is preferred when they have been walking about the house shod for an hour might make them feel rather awkward.