We have them under all of our upper cabs. I use them when prepping, but also as a night light on occasion if we are going to be in and out of the kitchen before calling it a day.

Task lighting is a more general term. UCL is one form of task lighting. A light over the sink is another, for example.

I put UCL under all my uppers, but I also anticipated doing work under all of them, too. (I cannot really imagine having counterspace where I don't do work, but then again my kitchen is petite.)

UC under all cabinets. IF I did again, might have put on 2 different switches- but that would have added to the price for a second transformer (already have 3 transformers.....(one for the inside cabinet lighting, one for the above cab/shelf lighting in the sun room and one for the UC LEDs)
I agree with Angie and others - I use just about every spot on my counters except maybe the 18 inch end to the left of the cooktop.

I have a galley and I really only do food prep on one side. The other side has a purpose but not specifically food prep. I was inclined to do the lighting around he whole perimeter, mostly for consistency and symmetry. But if I won't ever use it, I am wondering if I could save $ by only putting it on the side where I would need it.. What do you think?

Most of my uppers are over prep areas and I have undercab lighting everywhere except the cab on one side of the range (this is sometimes a landing spot but is otherwise not used). I have not missed having UC lighting there. The only other area underneath an upper cab that is not really used is in the corner. I needed lighting under one little part but not under the whole thing. My electrician recommended lighting that whole undercab area anyway using two bar lights in an L shape to avoid weird shadows, and he was right.

Now, in your case (galley with prep only on one side) I would lean toward lighting the whole thing. If you use that other side for anything that you would like to see clearly (opening mail, making crafts, etc) then you will appreciate the extra light. The symmetry and consistency argument doesn't matter so much to me -- my UC lighting is for function and I never turn it all on just to admire it -- but it is something to think about. But now that I've got UC lighting for the first time, I would say it's better to have it and not need it than to need it and not have it. Unless your lighting is prohibitively expensive, I'd go ahead and put it in. You could at least have the wiring rough-ins done even if you don't install the fixtures right away.

Space them to provide double coverage at the height from the counter to help fill in shadows.

About every 12 inches works well with pucks with an 18 inch counter to cabinet clearance.

Over a sink make sure there are two point sources or a long enough tube to provide even light.

Pucks are normally not more than 20 W each, so even five of them only amounts to a 100 W load.
The closeness to the task really helps also, a large portion of the light is where you need it

What do you use as UC lighting? The battery powered strips, pucks? plug-ins?

What do you use as UC lighting? The battery powered strips, pucks? plug-ins?

I don't think any of those are desirable. I used hard-wired strips.

We used GMLarc light bars LED - dimmable and absolutely hard wired. Batteries will drive you crazy!. If you are putting on a bottom trim - be careful not to place the lights to close to the strip - we had to move our back because of the shadows - now they are perfect!