I always have an A/B rig going-
1st rig is for rhythym, with my basic effect and reverb.
Then I switch to the lead rig, which always has a booster (I usually use the PB Confidence stomp), a little delay, and a volume boost (with wah in the first stomp on touch mode in case I want/need it)
I just can't get all of those things to happen at once any other way, so it's just easier to get them all on instantly in a new rig.
It takes some planning, but it's well worth it and the lag time from switching will never be noticed live (it's virtually nothing).
I used a Lexicon MPX-G2 processor with my my amps pre-KPA because it was the only MIDI controller that could do all 3 things with 1 foot switch click.
I should also say, I always use the same rig within a song- just use the booster in front for a little more gain.
I don't like changing actual amp rigs mid song, just too much of a difference in tone for me (not that anyone in the audience would ever know but I'm anal)