I used to try to buy a little of everything. Folders, fixed, gents, tactical, hunters, liner lock, framelock, Axis, lockback, balisong, auto, assisted, slipjoint, SAK, multitool etc...
But I realized that only the knives I really liked got much use, while everything else sat in a drawer. So I started focusing on what I really liked, which was larger tactical folders like a Sebenza or 710. I still bought some variety now ans then, though.
Then I got my first Bob Dozier. I sold off everyting else, and now Doziers are all I buy and use. Part of me felt bad to sell off and give away a collection of over 100 knives that spanned a lot of different styles and makers, but they were just sitting in my dresser. I see no reason to use anything but a Dozier, so those other knives ceased to be knives to me.
So after a year of collecting everything under the sun, and going through some phases, I have ended up just collecting Doziers. And I am more happy now than at any other point in my knife collecting/using.
I suggest buying what *YOU* like rather than trying to build a collection that would impress other people. I think my collection was more impressive than it is now. I think showing off a collection with a Sebenza, Strider, WHK, and a lot of different types of knives (balis, slippies etc...) is more impressive to other people than a collection of one type of knife that I really prefer. But I am happier now.