Do all Smallville episodes actually get completed before premiere airs on TV?

[PepsiMax]

So I was just curious whether all the episodes of Smallville are completed and finalised before episode one hits the screens? If that's not the case, would the writers then have a clear, detailed outline of what all the episodes will be and how the plot will pan out and if it actually makes sense? Or do they just write stuff as they go, i.e they have episode 2 done for example, and are currently writing (or thinking stuff up) episode 12?
It just seems by the way certain seasons panned out that it was all just written as they went. I don't know if it's completely true or not since I'm from Australia, but in UK they complete (or at least get the acting over and done with) all episodes of whatever season they are doing, and then air it. I think this is the case with Doctor Who series 5. And even though the wait was long, it was worth it because it's great television.
One disadvatage would be that fans would have to wait longer, but if it means a quality show, where episodes aren't always hated by fans, then I'm for it 100%.

Am I sort of correct here, or have I got it completely wrong? :D

[Supsfan]

They usually start filming in July(the season starts in Septemeber). By Dec they usually film about 14 episodes(and air about the first 9-10 of them). They go on break flming for a month come back and film Jan-beginning of April.

I am guessing when they come back to film episodes 15-22 they sort of know if there is another season or not by that point and the episodes get written accordingly

[ShiningPath]

Even though there is usually an overall plan for every season, sometimes changes to the plan have to be made in order to accommodate unforeseen complications, for example the unexpected death of Christopher Reeve caused a major shift in the planned story line.