It is great fun, no doubt, picking through tosai. And like palying treasure hunting on a public beach, we might find something interesting and even valuable but not likely 'treasure'.
Russ comments " aren't all fish sold off in grades?" . He knows the answer is yes but the selection itself changes with age. The tosai represent the identifiable male crop, the 'too good to throw away but no future fish', the colorful junk and the finely finished baby show fish.
Each one of these tosai is an education onto itself even if the lesson is " don't buy this kind of fish again!"
So in this regard, tosai can make you a better consumer and a more knowledgable koi keeper.
The Japanese koi breeder has an eye like a marksman. They also know their stock and know what they will look like next year and the year after. A crack in beni at the head might mean it will not be a three step beni coated fish. A flat head might mean the fish will not have volume as an adult in this line. And all the males tend to have a point at the tip of the fin or a very round fin. Or this particular pairing tends to throw off a percentage of young with small peduncles. Or even a speck of tarnish in this line of hikari moyo means they will be a mess by age three. This is not unlike playing black jack with the House's best dealer.
As the age groups go up, the name of the game changes. At age three the selection of for sale fish changes as the tateshita and males are mostly all sold out. Now we have definite representatives of varieties and the short comings are very clear. There is no perfect fish. And some fish that have negatives have very exceptional postives. This is now a different decision as 'change' can not longer be hoped for- what you see is what you get. And the hot tategoi is a 'screamer' at this age. The 'dealer' is also very confident about his 'cards' at this point.
Age four and the 'selection' is now dead accurate. Only the best is retained. You are now looking at the best 1% of a once 1/4 million spawn.
Selecting among these fish is clear- you pay more for best fish among many excellent fish. And the degree of variation may be in subtle differences of length, girth, pattern and yes, future from this point on. None of these fish are likely to crash and burn if you are buying from an established breeder. The only issue is quality and the ability to hold youthfulness. This age will 'come along', develop and delight- but major change is not likely.