For several years we used 1/2" and 3/4" poly mainline and used plastic fitting. Then we upgraded to 3/4" and 1" lines. Then we started looking into stainless fittings. There are two choices....plumbing and maple. The plumbing are thinner than the plastic and have a little better barb for more grip. The maple are much thinner and the barbs are much sharper. You can see the images I posted below are of a section of mainline I cut out. We have upgraded our mainlines to 1" and use all stainless fittings. They do not cost a whole lot more thank the plastic, hold better, creat less turbulence, and give less area for bacteria to grow.
There is not much relevance to pressured lines because the sap is moving downhill by gravity. Often times our mainlines are not full. They trickle down and any ledge in the line can upset the flow. If the pipe is near full and you do not have a dry line the transfer of vacuum could be interrupted or the flow of sap could have turbulence.
Mike
Mainline gauge.jpg
Maple fitting.jpg
Plumbing fitting.jpg
Tapping since 1985 (four generations back to early to mid 1900s). 200-250 taps on buckets and then tubing in the mid 90s. 2013- 275 taps w/sap puller 25 gal. 2014-295 taps w/sap puller 55 ga. (re-tapped to vacuum theory) 2015-330 taps full vac. 65 gal, 2016-400 taps 105 gal, 2017-400 taps 95 gal. 2018-additional 800' mainline and maybe 400 new taps for a total near 800 taps. 2x6 Leader WSE (last year on it) supported by a 250 gph RO.