Today, I read the following from the TTi Int'l web site (http://www.tti-intl.com  - then click on the AR-15 button):  

"Some shooters insist on having a second upper to mount on their rifle for different styles of shooting. If accuracy is not an issue, this is an acceptable practice. We advise against switching tops because the more you separate the two components, the more wear you add to their lugs."

Correct or bunk? What are your thoughts?

I have no evidence but I doubt removing the take down pins causes much wear. The fit of the upper to the lower isn't extremely tight anway. Also the sights and barrel are both on the upper so the effect on accuracy with a regular AR-15 would appear minimal. A match rifle might be different.

I dont worry a bit. I have a circa '93 Oly 11.5" . At the time Olympic put out "match" ARs..( lower matched to upper, for fit only if I recall). The upper and lower fit so damn tight I need a punch to get the pins out. This gun has been taken apart etc.etc.. thousands of times. This gun was owned by a SWAT operator and was an entry/duty gun so its well taken care of, it does show wear on the lugs but shoots like its new.. Better than my 20"A2 BM that has only 2k thru it... John

I suspect they’re referring to match rifles.

Constantly separating the upper and lower over a long period can wear things a bit.  I frequently clean and put a little lubricant on the takedown and pivot pins and in the lug holes to minimize this.

However, as c-wheeler mentioned, a tight fit isn’t really that important for accuracy  – it’s more like you just tired of things wobbling a bit.

Many folks separate the upper and lower when cleaning anyway.