Been thinking about this some more and reviewing the last session we played. We were effectively trying to escape a sinking submarine (don't ask, just go with it), lots of running, jumping, climbing, turning stuck valves etc. All great fun and like I said, very heavy on the ability checks. I didn't keep count but I must have cast Guidance something like maybe 20-30 times in just that one evening, possibly more. I did get kinda sick of saying it every time a roll came up though but as I also said, it's just too useful not too.
Too useful on paper that is...
So, of all those rolls, it felt like on roughly somewhere between two thirds and three quarters of them the Guidance never even got used. Most of the time because the roll initially succeeded, some of the time because it was so low an extra d4 wouldn't have helped. The rest of the time the extra d4 just didn't cut it and they still failed. Only on one occasion do I remember the Guidance changing the outcome from a fail to a succeed – just once. Of all those rolls and all that effort and all that rigmarole of saying "I cast Guidance on him/her" over and over again, only once did it have any tangible effect on the outcome of a roll. Incidentally, it was on a non proficient ability check.
My conclusion, casting it all the time turned out to be just a big faff for no real return. In the future, I will probably choose to reserve it for only the "important rolls," or to help out a non proficient comrade. Because honestly, it's more of a pain for the player (me) to keep casting it every time than it is a hindrance to the DM.