I was told not to clamp to the tubes when I got my very first "grown up" bike - Cannondale aluminum. I think for most stuff, it's not a big deal, but if you're exerting force, like tightening BBs or crank bolts, it's more of an issue. I've noticed in several of the 'bike build' video series, the first thing the do is put in the seatpost and clamp by it. OTOH, that British guy, BikeIt UK, clamps by the top tube, but he's generally working on old steel stuff.
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"Don't take life so serious-it ain't nohow permanent."
"Everybody's gotta be somewhere." - Eccles