Well, it depends on your strength and style and what you are looking for. I THINK that most people riding a fixie for training go with a lowish gear (for the particular cyclist) and use the fixed gear for cadence building. Or you can use a higher ratio and build strength. I'm not a strong cyclist so I started with a 42/18 and try to keep my cadence in the 90-100 range. A young friend of mine, who is a VERY serious racer, also has 42/18 on his because it forces him to spin like crazy to go fast. Today I put on a 17 because, as I explained to my LBS buddy, I am sneaking up on myself one tooth at a time. 18 felt pretty easy so I take a baby step to the 17 until that feels easy. I want to condition myself to gradually higher gears while maintaining cadence. I picked 42/18 to start with because I knew I could spin it 90-100 comfortably. I was concerned about the learning process and didn't want the extra stress of a higher gear.

I was ordering something from Harris Cyclery the other day and happened to get Sheldon Brown on the phone. He told me he mainly uses 42/15 and he has some hills to contend with.

Hope this gives you some ideas. It's really a personal thing. You can always change the gear. To me it seems more convenient to go with a larger chainring, ie 42 vs 39 for instance, and sprocket at first. That way you can increase the gear by using a smaller sprocket and you are always shortening the chain. Horizontal dropouts make chain tensioning easier but are not mandatory. You just may not be able to get the chain quite the tension you want. Have you read Sheldon Brown's articles on fixed gear?
Regards,
Raymond