always shot parralel cedars,just finished some 80/85"s ..tapered the back 9 inches and fletched them with turkey feathers,plain janes dipped in laquer,....nice shooting arrows, think i'll stick with this for a while..

I will be making my first wood arrows this spring and will be seeing how I like compared to aluminum. What is the benefit of tapered shafts?

Im no expert, but I had made a taper jig for my arrows and they shoot great.  The one thing I did notice, was I was over spined with the parallel cedars. once they were tapered they shot way better.

Tapering a shaft will allow for slightly more feather clearance coming off the bow. It also gives the arrow a more aerodynamic profile going through the air. Another benefit is it gives you more weight forward and because the back is slightly more flexible it comes out of paradox quicker too. Adding all of these attributes together can make a difference in an arrows performance.
I have some very accomplished tournament archers that have me make arrows like this. If you are talking the most accurate arrow it is what they want!  Of course, evenly matched and straight shafts are a must.

Another side benefit for split finger shooters. Since the taper lets you use 5/16 nocks so less finger pinch.

Has any maker ever made tapered carbons or aluminums?

There are tapered carbons on the market.

If you like to target shoot make a barreled shaft. Taper both ends. I use to make arrows for a champion English Longbow shooter and that is all he shot. So I made some up for my self years ago and man do they fly nice.

I have been tapering my woodies for years now.  I Happen to like a longer more gentle taper.  I shoot an arrow that is 28 inches long so I taper   14 inches . IF I shot a 30 inch arrow i'd taper  15 inches  etc.  
   This also produces a natural weight forward shaft .  BOTH   23/64  and 11/32  shafts down to 5/16 .

Putting a 8-10-inch taper on a cedar shaft will reduce the weight 30-40 grains depending on the shaft and amount of taper, i.e., from 23/64 or 11/32 to 5/16.  

That will increase the FOC by 1-2% depending on shaft length and point weight.

Such a taper has a negligible effect on spine, reducing it from from 1-3#.

It does very little to increase feather clearance.  For example, 11/32 to 5/16 is a reduction of 1/32-inch in diameter, or 1/64-inch in radius.  The height differences in machine cut and hand chopped feathers are greater than that.

To the extent the nock is smaller, it would increase finger pinch by causing the fingers to be squeezed even more closely together, but again the difference is so slight as to be unnoticeable.

Tapered shafts do recover slightly quicker than untapered shafts.

That's about it.  Not a big difference between parallel and tapered shafts, but several little things that potentially work in favor of the tapered shaft.  I've shot both for nearly 50 years.  I prefer the tapered.

They look cool too .... easily as important as FOC ...

Is there a sponsor that tapers a customers shafts?
Have a bunch of shafts that are borderline stiff for the bows I shoot now and would like to try some of these tapered before I invest in a taper jig.