You need to check what your coil manufacturer says as far as if you need to run a ballist resistor or not. I'm running a Mallory ProMaster coil. I'm running a .8 ohm resistor because I'm also runing a Pertronix Ignitor (1st gen). The pertronix requires 1.5 ohms of resistence. The coil has a built in resistence of .7ohms, thus the need for the .8 ohm resistor. Now this is dictated by Pertronix, not Mallory. If I were running a Ford Duraspark ignition, then I wouldn't be using a resistor at all as it is not needed. Again, this is due to the distributor, not the coil.
Check with your coil manufacturer as YMMV.
[Edit]OK, what I said isn't totally correct. After reading Scott's post, I should have added that it depends on the type of ignition as well. For example, with points, I would need to run more ballast as I'd fry the points if I ran it with just 1.5ohms of resistence. How much, no idea. That's where your coil manufacturer will come in handy. Pertronix was the one that dictated how much ballast I had to use to prevent their unit from going up in smoke. I could run my ProMaster coil without any ballast and have no harm come to it. It's my Pertronix that will suffer. As Scott mentioned, you have 2 types of coils. The advantage of an externally resisted coil is that it allows you to use it with different ignition systems by adding or removing resistence to meet the requirements of your ignition. Conversely, using an internally resisted coil with 3.5ohms as Scott uses in his example below would be excessive for my Pertronix ignition, resulting in sub par performance. There, clear as mud.