keeping each bunch a bit sparse and gluing the butt ends will keep a fly together through many a fish caught.
Ard's advise is about the best tip you'll get for working with hairs.
I've probably tied thousands of flies & jigs with bucktail. It's an excellent material, and not difficult to deal with, once you get used to it's nuances.
I don't bother attempting to stack the hair. It's not needed, and IMO doesn't add anything to the appearance of a fly. Actually, it doesn't look natural IMO.
Instead, I just try to even up the hair a bit, by holding the hair between my fingers & pulling free the longer fibers. Sometimes it takes a couple of attempts, but I've gotten to where I can do this fairly fast. This gives a tapered look to each clump of hair, which again IMO is more natural looking. (edit-posted before I saw Rips reply, we agree on many things!)
Here's a pair of Clousers I've tied. They're sparely dressed & approximately the same length. The tips of the hair are generally even, and yes there are a few longer hairs. Clousers, or any fly should not look like a paint brush. When these were tied, I gave the hair a few wraps of thread & a drop of head cement (Sally Hansens Hard As Nails). The additional wraps will work the cement into the hair, making them solid & durable. There is also a coat or 2 of Sally's as a final finish over the thread, eyes & hair at the head.
Here's another. Tied with a combination of squirrel tail & coyote tail. Again, the tips of the hair are approximately even. I use this fly as a crayfish imitation.
This hair jig is tied with coyote tail hair. Again, the hair is even, but not like a paint brush.
A saltwater Clouser, the tips of the hair gives a tapered appearance to the profile. Some baitfishes have this same profile. BTW, this is fox tail hair, not bucktail.
This large bucktail jig is tied very full. The tips of the hair are about even. Sometimes a fuller tied jig or fly is desirable, as it pushes more water. There are always times when sparse or full has it's uses.