is 440 stainless steel good for throwing knives?

Good steel ain't cheap, cheap steel ain't good...

If you're just starting out (practicing anyways), I would be less concerned about the material and more concerned about the style and shape of the blade to gain consistency in your technique...

When you start getting proficient for competitions and such... then I'd start investigating steel grades...

Anyways, here's a summary from the wiki:

Type 440—a higher grade of cutlery steel, with more carbon, allowing for much better edge retention when properly heat-treated. It can be hardened to approximately Rockwell 58 hardness, making it one of the hardest stainless steels. Due to its toughness and relatively low cost, most display-only and replica swords or knives are made of 440 stainless. Available in four grades: 440A, 440B, 440C, and the uncommon 440F (free machinable). 440A, having the least amount of carbon in it, is the most stain-resistant; 440C, having the most, is the strongest and is usually considered more desirable in knifemaking than 440A, except for diving or other salt-water applications.

Stainless Steel Throwing Knives

For throwing knives, 440 isn’t necessarily a bad choice. Does it list whether it is 440c or 440b? That matters somewhat. The advantages of 440 Stainless is that it’s relatively hard (for a stainless), it resists rust significantly, and it’s not so hard that it would shatter or break. On the down side 440 is notorious for not taking or holding an edge, but for a throwing knife, that’s probably not much of an issue. If you’re looking for something to train with, you’re fine here. It’s plenty of steel enough to stick in wood or other targets. Odds are it won't bend if you slam it into some wood. If it lands on concrete, eh, it's going to scratch and all that jazz, and it might hurt the tip if it hits first, but as has been said, you can use a file or grinder to get the tip back.