This is something I'm very conscious off when writing. My natural inclination is to write in English. Therefore to use "dressing". However, some think this pretentious, also if I used English terms across the board many would simply not understand. The English word for "leader" is "cast". Of course to differentiate between a cast and the action of casting is by context. Something which seems to be out of fashion. This may also explain the miss spelling of vice. For these reasons I don't use "fly dressing" , despite feeling that I should.

Having looked up the etymology of of the word "Dress" I came up with this

Origin:
1275?1325; Middle English dressen < Anglo-French dresser, dresc(i)er, to arrange, prepare, Old French drecier < Vulgar Latin *dīrēctiāre, derivative of Latin dīrēctus direct; noun use of v. in sense ?attire? from circa 1600(1)

The clue here is in the word "attire". To don attire is to dress. Is a fly not a hook (or other mount) attired with various materials to the purpose of attracting fish (or fishermen)? If so then the materials are the dressing, the fly is dressed, and the act of putting the materials on the hook is dressing. This also fits with definition 11.

"to trim; ornament; adorn: to dress a store window; to dress a Christmas tree."

Looking up "tie" came up with a different result.

Origin:
before 900; (noun) Middle English te ( i ) gh cord, rope, Old English tēagh, tēgh, cognate with Old Norse taug rope; (v.) Middle English tien, Old English tīgan, derivative of the noun; compare Old Norse teygja to draw. See tug, tow1 (2)

There is here no reference to thread, it is specifically larger ropes and cords.
Also,

tie [tahy] verb, tied, ty?ing, noun

verb (used with object) 1. to bind, fasten, or attach with a cord, string, or the like, drawn together and knotted: to tie a tin can on a dog's tail.

2. to draw together the parts of with a knotted string or the like: to tie a bundle tight.

3. to fasten by tightening and knotting the string or strings of: to tie one's shoes.

4. to draw or fasten together into a knot, as a cord: to tie one's shoelace.

5. to form by looping and interlacing, as a knot or bow.

6. to fasten, join, or connect in any way.

7. Angling. to design and make (an artificial fly).

8. to bind or join closely or firmly: Great affection tied them.

9. Informal. to unite in marriage.
10. to confine, restrict, or limit: The weather tied him to the house.

11. to bind or oblige, as to do something.

12. to make the same score as; equal in a contest.

13. Music. to connect (notes) by a tie. (2)

Notice that even here the modern use of "tie" as in fly tying has crept in at No.7. There are though, several references to "knot" and "knotting". The only knots I have come across in fly dressing are the ones used in certain types of woven flies, and Darrell Martin's micro cased caddis where a length of wire is knotted then wound along the hook shank. I know someone will have just said, "What about the whip finish?" Well the clue here is in the name. The Whip finish is a whipping not a knot. Someone else trying to be cleaver will have said "Ha then, there is the half hitch". Again a hitch is a hitch, it isn't a knot.

From looking at this it seems what we are doing is far better described as "fly dressing" than "fly tying". Despite this I do not expect anyone will change their terminology.

References:
(1) http://dictionary.reference.com/browse/dressing
(2) http://dictionary.reference.com/browse/Tie?s=t

Cheers,
A.