Dropping the odd clunker because you're developing an idea, going for that 'little extra' or are feeling inspired is cool, because its what live music is all about (IMHO).
Flatulent honkers due to being pissed, stoned or otherwise wrecked are just letting your band members/mates down.
I regret to say I've done both, and I know which one feels most 'noble'...I always feel that the art of improvisation is the ability to 'go with he mistake'. It seems to me that Jimmy Page has made a fabulous career out of it.
Phil Lesh of the Grateful Dead tells a story about a fistfight he had with Jerry Garcia on a night when they accused one another of playing the whole night in boxing gloves, not listening, fretw*nking etc etc.
When they heard the tapes back six months later, they released it as a live album...