Not really. A knife is primarily a single-edged weapon, and used for cutting more than stabbing. A dagger is double-edged and intended to stab. The first thing you have to decide is what the tool will be used to do. If it is only used to eat, you probably want a knife -- your noble fop can carry an ivory-handled knife with gold and gem inlays that is only intended to show his status at the dinner table, but your street rat might want something more practical.
Okay, so definitely dagger, yeah -- because he (or she) often has to defend against those who would see him imprisoned, or worse. You could stop there. “A dagger” forms a picture in your reader’s mind but doesn’t really spice the scene.
It is so much more interesting to know that your Malaysian urchin is wielding a kris handed down through his family, despite its curse. Or that your Italian noble has a stiletto stuck in his boot for emergencies. Whether the character carries a dirk or a poniard is just one more detail that can bring a flat world to life.
Does the character carry his weapon concealed in a sheath in the small of his back? He probably wouldn’t feel very comfortable with a foot long piece of steel sticking down his pants. And what about the ever-handy dagger in the sleeve? Probably want that one under six inches. There are as many sizes of daggers as there are types, so it is important to choose one that is appropriate for the character and the situation. A small child may use a long dagger like a short sword if her life depends on it. A large man may carry several three inch blades concealed for throwing. These are decisions to keep in mind as you write.
In The Luckless Prince, one of the elves carries a stiletto in his boot -- which comes as a surprise to one of the other characters. Collyn Silverbrook, on the other hand, has only his eating knife when he finds himself in a sticky situation -- and it is explained why. The villainous Norfulk Roderickson puts a sleeve dagger to deadly use. Making sure the right weapon is used for the right circumstance is one of the things that makes writing so much fun.
Details, details, details. They are the stuff art is made of. :)
For a list of daggers, go to http://en.wikipedia.org/wiki/List_of_daggers