Hi Karl,
In reality, only a relatively few instruments, like guitars, banjos, and keyboards are tuned ET. Most instruments, and the most common, like the violin family, winds and brass, are played in one form of just intonation or another.
If you buy a cheap Korg tuner, it comes with marks on the display to show you the two most common offsets for just intonation. So, if your drones would be tuned to D, then when you play D on your chanter the needle points straight up. When you play F# the needle points to the little triangle to the left of center. G is a hair flat, E is a hair flatter than that (-2 and -4 cents, respectively). A is a hair sharp, and so forth. You can look up just intonation offsets for all the notes.
When you actually have drones going, you can tune very precisely, and there’s an important reason to do so (because it’s really annoying to be out of tune with yourself), but without drones, there’s nothing to compare to, and very few people have ears good enough to hear a few cents this way or that in the context of a reel.
I don’t play UP, but if I did play them and if I didn’t have drones, I’d probably approach it the same way I do my whistles – as long as it doesn’t sound “bad” then it’s OK.
If you get to the point of playing with someone else, then you need to be more or less in tune with them. How much is a kind of interesting decision though. When we play highland bagpipes, they all need to be exactly in tune with each other, or it sounds terrible. But when I play smallpipes with my friend who plays harp, he tunes ET, and my smallpipe low G is 40 cents flat. I’m not about to carve the hole bigger to be in tune with the harp, so we’re just out of tune on that note.