I used a Hennessy Expedition and Hennessy Hex tarp for a year or two. In those days, my "year" was just a few trips in the summer months (mild weather). Along the way, I picked up the Hennessy Monsoon - sort of a hex tarp with Velcro® removable doors at the ends. The first time I tried it, I was at the northern beach end of Reid Harbor ( Stuart Island, San Juans WA) - so the wind had a 1.75-mile fetch (distance across cold water) before it reached my setup.
The first night I didn't have the "gimmick" doors attached - and I was ... cold. And a drizzle rain would blow in. The tree options required that I hang parallel to the wind. The second night, I attached the doors, and it made a world of difference. If your tarp doesn't have doors and you need some blockage at the tarp ends, it might help to hang your rain jacket, exterior side out of course, on the incoming rain/wind end.
These days, I purposely hang parallel to the wind because it presents the smallest surface area to the weather, and the trees offer some blockage. BUT, all my tarps have some kind of "door" - extra material at the ends of the tarp to partially or completely close those ends off.
Also know that there is no law that requires your tarp ridgeline to be level. It's probably better to set up so your ridgeline is at a slight angle, with the low end towards the incoming weather.
Like LowTech, I was also once in a situation where the wind/rain was broadside to my tarp. And I hadn't paid attention to potential drainage. So a "swimming pool" formed on one side (the tarp edges were raised a bit, like an awning). When I got up at night, my head brushed under the tarp, spilling all the water into my shoes. Now I purposely set up parallel to the wind when I can. But my tarps have end blockage sufficient for the predicted (plus a bit worse) weather.
I also have shock-cord (bungee) on the tarp edge guylines that allow it to deform a bit during gusts - spilling the wind - and spring back. And I pay more attention to, "Where will the water go" drainage.
But these aren't mechanical, "Always do this" rules. Large storms usually don't magically appear. Usually, there is some warning, some prediction, some local knowledge (even if that local knowledge is, "sometimes storms magically appear."). If I know it's going to be a mild night, with no full moon shining right in my eyes, I'll probably keep the tarp strung up, but in its snakeskin, so I can see some stars.
