Some you do, some you don’t even if you know the dog has a terminal illness. One of mine was diagnosed with lymphoma after I felt a lump in his neck. He’d shown no signs of being ill other than the mass and because he was up there in age, we all decided to let him alone rather than put him through chemo etc. He had another 6 full months of great health (to the point I’d gone into denial) until his final couple of days when, having called them in for their suppers, he didn’t come with the others. I found him collapsed and got him straight to our then vet. He was given an injection to see if that helped but sadly it didn’t so the following morning he was given peace.
On the other hand, we’ve had some who just deteriorated to the point that as we don’t do ‘lingering’ we had to let them go. For me it’s all about quality of life so if there is little to none, the down days outnumber the up days, then it’s time.