It's all about balance. There are a couple of different aspects..... some folks, usually stock removal makers, will taper the tang. Those who forge will tend more so, to install a distal taper (thickest portion of the blade is the ricasso area, and the blade tapers fore and aft from there)
In the case of just tapering a tang, it lightens the handle end, and will often give the knife a "lighter" "faster" feel. Those same things apply to a full distal taper, but the tapering of the blade also lends additional benefits.
I've also found that the tang after tapering the tang it needs to be flat otherwise getting a good fit is more difficult. I've taken to using the layout dye or a Sharpie to make sure I get rid of any high spots etc. before attaching the scales.
Drill holes first, definitely as has been said. Then decide on the thickness at the butt end. Grind the edges then grind it flat. It’s really easy to get a convex shape ... but it’s gotta be flat. I’m sure you knew this already, but I finally halfway know something lol. I takes some planning ahead, but you got this. Jay fisher states, on his website, (paraphrasing big time) that the tapered tang is the way to go because it’s the correct way to lighten the handle area to achieve the balance that Ed was talking about. Jay claims that Swiss cheesing the tang results in a weaker knife... maybe so? I personally think that a tapered tang looks better but that’s just an opinion. To me it looks kinda “rookie” to have a tang that’s 3/8 wide throughout... lol. But some people like it.
Just my two cents.... probably all wrong lol.
Distal tapering the blade and the Tang gives a excellent visual when the customer holds it in their hand. The balance other’s have mentioned—————————————Especially when you have added in some colored fiber material to accent the space in between the scales & the tang! ———The Late Mr Bob Loveless said, “I use Red because it looks like a little bit of lipstick on a pretty girl “...........::........................................... I use Red, Black & Pink for the ladies on my Culinary knives..
When you hollow grind a blade, you're taking off a lot of the weight in the blade section. That tends to make the knife handle heavy. Tapering the tang helps with the balance. Also, it looks nice.
The holes in the tang are drilled prior to tapering, but you're correct that it makes drilling the scale holes more complicated; you have to have some way to hold the knife blade level while drilling. If you just put it on the drill press table with the scale flat, the blade will be angled upward and your hole will be crooked. I usually just put a drill bit under the end of the scale and eyeball it to see whether the blade looks level. The exact size of bit depends on the handle length and the amount of taper.