White smoke = no bueno. I just did a head gasket on my Mazda and Acura this week. Not to fear though, it's not that bad to change. If you're doing the head gasket, do the valve seals for sure! You can buy a complete head gasket kit on the cheap which includes everything you need (including seals).
Sounds like yours isn't too bad yet though. My mazda blew a huge freakn cloud of white at start up! Now it's sitting pretty. If your valve seals are bad, you'll know right away when you take the valve cover off. Milky looking oil means water is getting into the cover. If there is no white milky stuff, the seals are more than likely ok, but you should change them anyway if you're doing a head gasket. Takes all of an hour (at most) to replace them once the head is off.
FYI, I don't think it's possible to change valve seals with out pulling the head anyway. I've never tried, but I don't see how it can be done. The valves would fall in unless you catch them some how with the piston or something. Complete waste of time to even try I would think. Just pull the head, change all the gaskets, rings, and seals and be done with it. It'll run like a champ!