If your world spawned with the Crimson instead of the Corruption, Corrupt Seeds can be used to grow corruption in your world as well, useful for obtaining certain items that can only be dropped by monsters in the Corruption.
Placing Corrupt Seeds close to your Spawn point is dangerous as it will spawn Monsters from The Corruption.
If you plant Corrupt Seeds near the original Spawn or a Bed, Corruption will be placed 200-300 blocks away from it.
Planting Corrupt Seeds in itself may be dangerous because corrupt thorns will grow.
If you are playing on a large world, The Corruption will not be close to you.You may try to place corrupt seeds at 100-200 blocks away from your bed.Although in the 1.0.6 update you can place corrupt seeds 40 blocks away from spawn.