If the valve seals are bad the oil level should be dropping. I replaced seals on Toyota's that did the puff of smoke when started or after idling a while. You can screw a compression tester hose in the spark plug hole, pressurize it, and use a special sping tool the remove the springs to change the seals.
i have seen a puff of white smoke at begining of shift or after several hours beeing stoped. have found Like eduard says : bad valve seals.
Also a bunch of smoke, like a cloud comming out (oily smelling), and after "less hair at my head". i found excess of oil at rocker cover, beacause of a worn rocker shaft.
might be valve seals...
does it only happen after sitting overnight, or every time you start?