Yes it may just be a mountain made out of a mole hill to be fair though I must point out that Tessio was in a physical position to offer his right hand too. There is also a slight stand-off before Micheals reaches to accept his handshake.
Clemenza didn't shake Micheals hand at all.
I've never paid much notice to that particular handshake, but I think it could indeed be meant as a sign and give the (wrong) idea that it would be Clemenza who turn traitor. But the 'left' handshake could also be a secret hint that Tessio would be the traitor.
