dpb wrote, on Sun, 28 Sep 2014 12:04:29 -0500:

I don't believe there's any reason to think the environmental thermal
cycling has any chance of loosening those sufficiently to worry over
from that standpoint.

I don't know. But, safety is cheap, if you know what to do, so I'm not
against the advice at all. This has to handle kids and adults, and has to
outlast us.

They've been installed w/o lock washers for some 60 yr in
SW KS which is quite extreme in both temperatures and particularly in
changing in extremes over very short time frames relative to CA redwood
country. Not a single one has come loose on its own in that time.

This is great information, as your environment is the same as ours!
We're on steep hills in California, in the redwoods.

What I'd suggest and use would be
a) at least two/ location, preferably three, and

You can see that we used *eight* on the connection he
https://c3.staticflickr.com/3/2944/1...3de04150_c.jpg