Reason i say the does it do it when its been sitting yada yada - Gives time for oil to seap past the valve guides, and sit on the back of the valve or inside the combustion chamber, dependng on where the motor stops. So, when you start it up, if its a ploom of smoke, then it goes away, thats usually the problem. If it keeps smoking while ure driving - its either rings, or very badd valve guids.
Its cheaper to do valve guides.. kinda.. take the head off and valves out - replace the guids; (motor in job)
Rings - well, yeah motor out start again roger type thing..