I used to use smoothing all the time for all IRs, because I had these harsh frequencies that took too much EQing to dial out otherwise. But I ended up discovering that the cause was my guitar itself, and I was able to mitigate it by lowering the height of the resonant peak of my pickups (by installing trimpots in parallel to my volume pots and dialing the trimpots to mitigate my guitar's obnoxious pick attack). I now find my tones to be easy and quick to dial in, with both static IRs and Dyna Cabs. I find that the guitar is just sweeter and more pleasing overall.
I used to get great tones with smoothing, but they always were so much less natural (although not harsh), the extreme amount of work I had needed to do with EQ without smoothing I traded for extreme work to make the tone sound more natural with smoothing! But ever since making the pickups sit better with my overly bright guitar, I'm finding it not to be necessary, and I'm getting way better results. Just a thought, and of course my situation may not translate to yours or to the reasons others may use smoothing.
