That is common. The reason is when you get tired, or have to climb an extreme amount, you are going to want a lower gear or number of gear inches on the freewheel side. What gearing are you running right now on the freewheel? If it is 46/16 I suggest getting a 16 tooth fixed cog. My reasoning would be you do not want to be riding a 46/15 or 46/14 as a new rider. That would be approaching/over 80 gear inches.

Now I am not a pro on this, as my first fixed gear will be arriving via UPS today (messenger from BD) but I think my logic is straight. I will probably be buying a smaller chainring or a bigger fixed cog, as 78 GI is going to be fairly high for me.