Sheldon don't tell no lies. Look at his chart when choosing your freewheel and cog, but if you already have the parts and there's a difference, make the cog happy and let the freewheel be off, don't split the difference. You care more about the cog, since a derailed chain on the fixed is way worse. For a while I accidentally had just about the most off combination you could get, and the freewheel was no worse off than pretty much any gear combo on a geared bike.
Also, be sure you get a BMX freewheel remover, not the regular 2 pin one that fits old suntour multispeed freewheels, or you'll have to grind it to make it fit. Not that I've ever had to do that, of course, I just, um, heard it from a friend