Assuming you know the OLD you need for your hub and buy an appropriate wheel, the dishing of the wheel and the axle spacing should be such that the cog will be placed close enough to the drop outs that you will hopefully not need any crazy solutions (IE going SS and getting a chain tensioner or something along those lines). Whereas say if you were to suicide hub a road bike wheelset (freewheel type of course), a 5 speed might have the cog outboard enough without redishing the wheel or respacing the axle, worst case scenario for that would be something like a 7 speed freewheeel compliant wheel, without the spacers being adjusted or the wheel redished.. you could literally wind up with your cog 3 inches from the drop outs, now picture the bend the chain would be making from the crankset to the cog.. you'd be lucky to keep the chain on hitting a light bump like that.

Example of a good chainline:

The fixed gear cog would be sitting about where the lowest gear on this cassette would be, to give example of the bend.

And please do not try a suicide hub.