I try to fit cards in with what is already there.... though I do also try to include cards I like as well... I think in "early" deck formation it's something I like, but later - it has to work with what is already in place.
One thing I learned from a good player a long time ago is that every deck should have at most 2 or possibly 3 things going on in it... and every card in it should accomplish one of those 2 things.... if it get's too scattered, it just doesn't work... Of course, if it meets it's nemesis it's also done for... but you just have to hope you don't meet it's nemesis...