Thoughts, but not definitive answers
Check stay trivia- any boat with parallel shrouds, like our Olson's, has no built-in way to keep the mast from pumping forward when the bow collides with steep seas. i.e. we do not have "double lowers" like the BK-designed Ericson's.
Then, you have to consider just how much stiffness is designed into the spar section that we have. Our boat has the Tall Rig and also the check stays that are led to turning blocks just in front of the aft mooring cleats, and thence to the secondary winches. While this was the way the factory did it, I understand that some other boat have those 'runner tails' cleated differently, and some may even have the double-fiddle block arrangement with a cam cleat.
I was told by rigger friend, when we bought our boat, that these were not needed in the less-harsh waves on our inland waters.
The stock runners were SS, and had an eye several feet above the deck, and a half inch line then goes on back as a 'runner tail'.
Back when we re-rigged the boat, these were replaced by ultra low stretch lines, also with eyes to connect to the tails.
Since acquiring the boat in '94, I have almost never rigged the tails, leaving the upper check stays secured to the turnbuckle area on each side with light lines.
A few years ago, when we went North for over a month, up and then back down the WA coast, we did have them both engaged, and snugged up. While never in sustained big seas, it just seemed prudent out in the ocean.
Another site member with a standard rig on his O-34, told me he has been sailing and doing overnight races off the SoCal coast for about 20 seasons, with no check stays rigged.
I would advise being conservative and using them when the seas seem appropriate. Tacking and jibing will be more labor intensive.
Harking back to our last (rough) trip over the Columbia bar, it was comforting to have them tight when the boat spent a couple of hours ... going up 7, down 6, 8 to the right, and same to the left, rinse and repeat. :eek:
If nothing else, it's two more solid lines to hang on to, along with a loaded-up mainsheet.