Thanks for the responses, all.
Personally, I think the “check during a lull” is generally a tad rude - with some exceptions (ailing relative, evolving plans for later that day, etc.) But I’m pretty resigned to the fact that a large (growing?) portion of people (of all ages) thinks that appropriate.
I recall one instance that bothered me a bit - and caused me the re-evaluate things a bit. We went on a ski weekend with 2 of our kids and their fiances. One couple was on their phones incessantly. Since we see each other infrequently (living hundreds of miles apart), we thought they could check their phones during the majority of the day that we weren’t together. And their close friends could have understood that they were on a vacation weekend with parents/prospective in-laws. We went out for a nice dinner, and were a little dismayed when in each of the photos we took around the table, one person had their phone in their hand. I assume just a habit, with no ill-will intended. But somewhat unfortunate and rude IMO.
My second question concerned when you send a text to someone whom you KNOW checks and answers their phone very regularly. You can be pretty sure that, unless they are asleep, on a plane, or at a movie or something, they check their phone at least every 30 minutes - if not as soon as it signals. And they tend to respond immediately - even to texts about which they will say, “nothing’s up.” I assume that such folk realize that they are sending a message to the folk whose texts they DO NOT respond to somewhat promptly (within a couple/few hours?) Essentially screening one’s calls.
I tend not to be the most prompt responder to texts. I’m unable to during the majority of my working days, and my personal habits (flaws?) are that I do not always have my phone with me/charged. But I do check my texts/e-mail at least 1x a day, and respond to ones from people who might expect/appreciate a response. If I know your regularly accept calls from folk on relatively mundane topics, if you do not accept mine - even to respond with a placeholder “k” - it is difficult not to think the recipient thinks none too highly of the sender. Not looking for grudges to harbor - just trying to navigate the changing seas of tech-iquette, and assess/re-assess personal relationships.