I don't want to use the word "precedent," but it happens as often as it doesn't.  Calling for a halt, I believe, costs money. 

To avoid this cost, tiny junior mining companies will often announce outside of market hours.  Why I see drilling news releases during market hours, I don't understand, but it does happen, though quite rarely.  Maybe it happens more often with Canadian companies; I don't recall. 

In any case, I think releasing good news during market hours misses a promotional opportunity.  And, in my opinion, getting the best bang for the buck on good news comes from calling for a halt, "pending dissemination of information," as I believe it is termed.