Here's some physics for the no intentional leak camp to think about. One inch of vacuum will lift sap one foot more or less. Along with that thought, vacuum does not transfer through a liquid. Considering those facts assume you have a sap ladder 12 feet high with 4 tubes, the system is tight with no leaks and 22" of vacuum at the releaser. Sap flow just gets started for the day, not much tree gas is being produced. All 4 vertical tubes fill with sap and slowly climb. When the tubes have filled to a height of 10 feet you're using 10" of vacuum to do that, how many inches do you think you'll have beyond the ladder? 12 inches. Now if you have enough tree gas, existing leaks or an intentional leak injecting air the vertical tubes keep percolating empty, you're not lifting a solid column of sap and vacuum level beyond the ladder will stay almost as high as at the releaser all the time. Watch your ladders, if all the tubes are full at the same time physics dictates that the vacuum level beyond the ladder will be lower equal to the height of the sap column. If at least one tube is always bubbling along then you're fine.
Tim, the same thought applies to laterals running up hill. I've got a few and yes they work to get sap to the main and are better than not tapping those trees, but you'll have higher vac to them with a properly functioning ladder than you will with a lateral full of sap.