Jpmcwjr: are you referring to straights only? (Again, the pipe cleaner test doesn't make much sense to me in most of the big Pete bents, because of the moist trap gap)
I'm with you, I didn't know it was a "correctness" principle. Made me look at my straight pipes (Dunhills, Savinellis, a Peterson 106) and I see (non-problematic) gaps. in fact I'm suspicious of this rule - with heat and moisture and all, there is some material expansion that could occur. A tight fit here may not be achievable or desirable.