The major reason to want to use a pulldown resistor is if you have to momentarily pull the input HIGH with a switch.
I don't much like pulldown resistors. In TTL, you have a wide window (or target) to shoot for when pulling up, i.e., there's a broad range of voltage that TTL considers to be a logical HIGH. On the other hand, when pulling LOW you have a very narrow window as a target -- only 0.8v as Ron mentioned. It takes a much lower value of pulldown resistor to get that LOW low enough that you have a guaranteed LOW that won't be susceptible to noise at the input or on the power rails. Where I can use a 1K or 10K resistor to pull LS TTL to a HIGH, it may take less than 100 ohms to get it to a LOW.
If you have to have pulldown, you're better off driving that input with an inverter and pulling the inverter's input to a HIGH.
Dean