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Shroud Lines
From Knacke (6-84), : "all lines and tapes are [traditionally] measured under a preload. A preload of about 1% of the ultimate material strength is required to align all fibers in the woven or braided material before the individual fibers are stretched." Furthermore, "[it] is practical to let all fabrics relax for a short period after rewinding from the storage spool before measuring the material."
From Poynter (8.1.5): "Because of the uncertainty of canopy loading due to non-uniformity, suspension line systems should have a designed margin of safety of 2.0. Lines lose about 35% of their strength due to sewing and the attachment of fittings."
Vent
Using a vent helps stabilize a parachute and eases construction.
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Heinrich and Haak, Stability and Drag of Parachutes With Varying Effective Porosity
Dan Poynter, The Parachute Manual: A Technical Treatise on Aerodynamic Decelerators, Third Ed.