Well, that depends a lot upon your room. Certainly EQing a sub will affect the response outside the "sweet spot," but the general idea (at least for me) is to optimize the response for the location where I sit. The laws of physics pretty much dictates that it's almost impossible to get truly even, flat response thru the entire bass range in a smaller room, at least without staggering amounts of room treatments (ie anechoic chamber).
Using room treatments will get you a bit more even bass throughout the room, but even this won't achieve total balance in all locations. A complex system of wave interactions occurs at intervals based on room dimensions & wavelength; some of these interactions reinforce while some cancel, creating peaks and nulls. By EQing out the peaks (nulls are very hard to deal cure with EQ), you do get smoother bass in all areas, although this sometimes results in even weaker bass in null areas.
