I like it both ways, and i do notice a difference, but this might be because of things other than the ir in the cab block, like the high and low-pass filters, motor drive, saturation, and a bit of eq. In other cases its because I'm putting it before more than just the cab. For post punk-y sounds like the smiths, the cure, the chameleons and all that type of stuff, I like putting it before even the amp block sometimes, and sometimes before some of the modulation as . This will obviously get a much grungier, sometimes pretty harsh sound, but if you tune it in right, you can get something like those pleasantly screwed up, "over-produced" (In quotes because i don't think theres a set amount of production one should do) 80's+ tones. I'll usually do this with mid-gain and clean sounds, I don't think I'd recomend it high gain, except maybe for some one-shot crazy fx.