On my race bikes the fuel always gets mixed the day of use... but that's more about fresh gas than the oil falling out of suspension, so I really can't comment about that. I have never had any visible separation but it can happen under the correct conditions.... another possible plus for the newer blends. Even the mighty A747 was a blended oil, not a straight castor base (who knows how much actual castor oil remains in it), and what little was left in the racer when I tore it down for inspection was still in suspension.
The old shake the bike side to side may be more of a nervous habit than anything else. The good news is most oils are colored such that they will dye the fuel once mixed- any separation will be clearly visible as dark blue/green/purple/brown sludge in the bottom of your tank.