I encourage sailing friends to downsize their cordage a bit. Using the modern high strength low stretch options it would be nice to not see those situations in which a line barely makes it through a block, won't stay in a cam cleat , over rides a winch, or runs poorly over a mast sheave, because it is too big. I also call into question the standard charts because they generally assume cordage which is cheap and larger than it needs to be. I think it's best to look at the strength needed and select cordage with the strength and stretch you need. And also account for UV resistance, float ability and other options. Yes, you can spend more but the pleasure and safety of having running rigging that runs smoothly is worth more. I know there's a lot of talk about using oversized line for the "Hand" but I think that's overblown. I'm not talking extreme here. Just a little smaller.
@shemandr I mostly agree with you, however, line selection has become more complex as there are issues in addition to strength and hand. I've learned this the expensive way.
When I bought Second Star she had wire-rope halyards. I replaced them with with low stretch NER VPC. and downsized the diameter of the rope portion from 7/16 to 3/18". All was well for several years until last summer when I motorsailer to weather in 20+ knot winds. The additional load on the main halyard caused it to stretch and start slipping in the rope clutch. We'll be ordering 7/16" VPC soon.
From that experience I learned there are 2 kinds of stretch, fiber stretch and lay stretch. Another factor to consider is the rope clutch or self-tailing winch size. Rope will stretch in 2 ways; 1) the yarns that a comprise the rope will stretch; and 2) the the lay of the rope will compress without the yarns compressing. I think the latter happened on my main halyard and that caused the halyard to slip in clutch.
Line that has a parallel core, like Sta-Set X will not stretch due to the parallel core. Other lines that have a braided core will. Think of the Chinese Finger Trap. When the ends are pulled the diameter gets smaller. The smaller diameter does not allow the clutch or self-tailer to grip the line as well and it slips. The yarns don't stretch, the structure of the line compresses and stretches. Parallel cores won't collapse like braided cores
APS has videos that show this. Here's one that shows the Samson XLS braid:
It is also important to consider the application. In some applications, stretch is good. For example, a traveler control line has to absorb a lot of shock loading in the case of an accidental gybe. A stretchy line, like Sampson XLS will absorb some of the shock protecting the traveller hardware. In other applications, moderate stretch is OK. Jib and mainsheets are frequently adjusted due to changing conditions, a little stretch is not a bad thing.
Another aspect is the length of line that is subject to stretch. Consider a Genoa sheet, the while the line will be 1.5x the boat length, the length of line subjected to high load conditions will be much less. On my 36' boat, when we are hard on the wind, the loaded length of the line is maybe 20 feet, stretch is just not an issue, I can always crank the sheet in a few more inches.