Every situation is different... I know three couples, announced they were engaged, and it was a couple of months before they got the ring. One couple still don't have a ring...
My h2b - proposed to me, but had the ring - we announced pretty much as sooon as we seen everyone... Personally I would prefer to have the ring with the proposal!
I got engaged but didnt get the ring til a week or so later - we waited to announce it to family etc until I had the ring. But if for any reason there was going to be a longer delay between proposal and ring Id say we would have told.