Using impulses isn't necessary, but I will say that, in my experience, bypassing the POD cabs and using impulses does give it more depth and make it sound more realistic. My POD X3 sounded much better through a real cab than it ever has direct. A lot of great sounding albums have been recorded with the stock cabs, though.
Drew is probably right in that it might be better to think of it as mixing any other amped, miced guitar tone rather than specifically dealing with it as a POD. Mess with the EQ knobs until it sounds good, find a good cab/mic combo, don't go overboard on the gain, EQ out bad frequencies in the DAW, etc.
If you're considering using impulses, though, have you considered using plugins for amp simulation as well? There are free
amp simulation plugins (like the ones made by LePou
, for instance) that are a lot closer to the real thing than Line 6's old XT/X3 generation modeling. If you have a proper recording interface, you'll also get a fidelity boost by not using Line 6's AD converters, which leave a bit to be desired.