Most devices, such as the 555 and all TTL outputs, can "sink" more current than they can "source" and as such, it's usually better to connect the LED's anode to the positive supply and the cathode to the IC output through the proper value current limiting resistor. It's a little easier on the output electronics.
In logic cases, that may often "invert" the desired output indication, so an inverter between the device output and the LED would be in order.
Dean