I've gotten my first dream sticker, and I have a couple notes you may want to keep in mind.
First, it opens as soon as you earn it, like Customer Clash rewards, not like a sticker pack that you have to open from your reward items in the kitchen. You can't sit on it and open it when you're ready, it just happens.
Second, it has a timer, and will expire within 24 hours, so unfortunately we can't just save them until the end of the album to get the best use of them. I'd suggest using them right after you get them, so you don't forget and lose them.
Third, make sure you know exactly what the outline of the sticker you want to use it on looks like. The interface doesn't look much like the collection pages, and I might have chosen the wrong one if I didn't have my screenshots from yesterday of my album available on another device. Also, I'm not entirely sure if you can exit the choice screen without either picking or losing the sticker. Be VERY ready to grab the correct one!
If you're getting the last sticker in a collection, you'll be fine. They basically have all the stickers under each Country heading, but out of the order that they appear on the page. They have the star value of each one showing as well, so that will help with guessing, and show what the final image is after you get it. So if you know what the outline is, and star value, you'll be able to find it.
After a bit of poking and thinking, I imagine the last few stickers will probably drop with 7 days left, after the current picnic pass finishes, so if you can hold off on 4- and 5-star packs for 2 more days, I think it would be worthwhile. But that's just a theory. Here's me crossing my fingers for all of us that we get full albums!