I do most of my post-processing in Adobe Lightroom 3, which offers some really nice noise-reduction algorithms. However, I make it a point never to apply noise-reduction unless there is noise in the photograph.
High ISOs are a no-brainer candidate for noise-reduction, but like you, I rarely shoot any higher than ISO 400.
However, noise can pop up for other reasons, as well... usually as a result of post-processing. For example, boosting the exposure, brightness, or fill light in a photograph can sometimes reveal color noise in the brightened areas. Adjusting saturation/vibrance, hue and luminance can introduce luminance noise, as well.
In these instances, I will zoom in to a 1:1 view and tweak the noise-reduction settings until the affected areas have smoothed out sufficiently.
But, by and large, I do whatever I can to achieve my desired result without having to bring noise reduction into the photograph. Even though the NR algorithms may be very sophisticated, the fact of the matter remains that they necessarily soften the sharpness, details, and contrasts of the photo. This is negligible in photographs that genuinely call for noise-reduction, since they stand only to improve as a result. But in other border-line cases, it means staging a balancing act... using only as much NR as necessary without having too much invasive impact upon detail and contrast.