exactly,, you can have a air tight joint at the time of install but the paint will crack no matter what.Our standard procedure is to caulk all seams and joints with painted trim regardless of fit. Even if they aren't open at the time of installation, wood always moves. Caulk is flexible, and will move with it, keeping a nice tight seam.
We never caulk on stained trim, but always on painted trim.
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