i had this same question this morning, even at WOT my bov is hissing like crazy...i dont think thats right. and at idle the bov is shut.

 

@ idle you don't build boost, sounds normal

 

BOV is opened whenever the vacuum pressure is greater than the spring pressure holding it closed. You could probably increase the spring pressure so it's closed at part throttle but it may not open as quickly for shifts. If it's open in boost something is wrong with the BOV or how it's setup. If your unsure have your tuner check it out.