My 92 Chevy Astro has had a puff of white smoke (on start up) since it had 50,000 miles. It now has 136,000 miles and still runs like new.
The problem with my Chevy is the valve seals. The fix would to replace only the valve seals - a complete valve job is not required since it doesn't burn oil.
The valve seals prevent oil from seeping into the cylinder while at rest. If the valve seals leak a little bit then oil would get past the valves and drip into the cylinder. Keep in mind that with the engine at rest some of the valves are open. It would take only a few drops of oil to create a puff of white smoke on start up.
I would have done this myself except that the engine on a van is difficult to access. I don't belive that any damage is being done to the engine since the puff of white smoke only lasts a few seconds and the spark plugs are not being fouled by oil.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles