I would guess that you're thinking of how the grain rifts are pointing.
Typically, we want the rifts pointing to the tip of the arrow on the top of the arrow.
So if you're using 4 fletch, that means you have a 50/50 chance of putting the arrow on "upside down" and risking impalement, dismembermant, and death on your bow hand... right?
A few months ago I posed a question here about how important that rift orientation is. The consensus on the thread was that most of us wood arrow makers follow that nock/rift orientation. However, none of us had ever seen an arrow break along those rift lines and head either up and away from the bow hand, or down into it.
The decision seemed to be that it probably doesn't make any practical difference, but we'll still do it just for the sake of uniformity, if nothing else.
Now, to further confuse you... if you use an index nock like the Bohning Classic, you can easily have the arrow oriented properly for the string before it fully leaves your quiver. I have a whole bunch of arrows with no obvious cock feather and it's no trouble at all to put the arrow on the string correctly without even looking at the arrow.
Also... if you nock a 3 fletch arrow backwards, go ahead and shoot it. Some bows actually like that better and even if that's not the case it's rare that you'll really see poor flight with the cock feather in.
Not that there's anything wrong with a 4 fletch arrow, if that's what you like.
Guy
Guy