Tailoring Your Army - A Newbie's Guide to Avoid Being Punched In The Face
'Tailoring' is the practice of attempting to guess what your opponent will use in the next game that he plays, and then deliberately fashioning your army so as to specifically exploit their weaknesses and blunt their strengths.
There are different degrees of Tailoring, from the very vague ("my opponent is playing Tyranids, which are traditionally famous for including lots of fast moving close combat specialists; it would be a good idea for me to take some units that can shoot them before they get too close") to being deliberately specific ("Tomorrow I am playing James, he always uses 3 squads of Green-Horde Orks led by Ghazghull Thraka all riding Trukks, and backed up by 2 Dreadnoughts, I will pick units V, W, X, Y and Z for the specific purpose of destroying those enemies respectively and at allocated times and places").
According to the Rules of the game as they are written, it is not illegal to tailor your list to your opponents'.
It is, however, somewhat against the spirit
of the game to do it without your opponent's consent. It's no fun being steamrollered off the table in 1 turn by someone who turns up and surprises you with the express intention of destroying your army as quickly as possible, but at least if you both agree to do it to the the same degree, then you both has as good-a chance as each other as your double-bluffs and double-guessing begin to balance out.
In short, there is nothing inherently wrong with tailoring your lists to be more successful against a given opponent. Be aware, however, that the more specific your tailored lists are then the more likely you are to be called a poor sportsman or even a cheater (particularly in competitive play, where tailoring might require knowledge that can only be attained through underhanded means), either of which will ultimately end with people refusing to play against you.
And where's the fun in owning $500+ worth of toy soldiers that you can't use, for the sake of taking a few risks and trying out things that only might
work in your army, and thus make it a competitive game rather than a foregone conclusion?