The only time I would suggest blowing a call without actual seeing birds, is in the fog. On a day when there is visibility, it doesn't do any good just to sit and blow a call! If you can't see birds in the air, there probably isn't any birds close enough to you to hear a call. Just keep the thing in your pocket until you can see birds....then pull it out and work them. Your decoys do more to attract the birds at first then your call ever will. I am a firm believer in the idea that less is best when it comes to calling. Just give them enough to keep them coming. There are probably more birds flared from hunters by improper use of a call then anything else. Good luck!
Pilots love to fly "HIGH"