Touchups are necessary. Sometimes the nail head needs touching up. Sometimes there is a scratch on the board. Sometimes where you cut will "splinter" and the cut needs a touchup. Like I said touchups are necessary. If your not touching up, you may be leaving points of water entry into the siding substrate, which is not good...
BUT touching up a spot here and a spot there takes no where near as much time as hiring a painter to paint the entire exterior. Also our siding installers caulk all butts where one piece of siding meets another piece of siding or trim. This is necessary for a long lasting system and because I am giving a warranty, I am not leaving this up to a 3rd party painter.
If you use pre-stained, the caulk color matches exactly to your trim or your siding, which ever color you order. When I say matches exact I do mean exact. The pre-stain facility dyes the caulk with the same stain they use for staining the siding and/or trim. You don't have to touch up the caulk.