As somebody who is about to start shooting color slides...i am all ears.
Though, i have been told it is best to er on the side of underexposure.?
Well, yes, because slide film doesn't handle overexposure well at all. If you've blown (overexposed) a scene or portion of it, any information contained within that area is lost. On the other hand, an underexposed image, while perhaps not very usable on first glance, will likely still contain quite a bit of information, and often this information can be coaxed out of the image, especially with the use of some good software once the image has been digitized (gasp!).
But really, underexposure can also result in ruined photos. So it's best to just try to be as accurate as possible with your exposure and also to be selective about where in the scene you're applying your exxposure to. You can have dark areas and very bright areas in a scene, but you don't want either to overly affect your exposure. Me, I have found that my highest number of keepers comes when I look for that mid-tone, that 18% gray, within the image, and that's what I meter off of.
Benjiboy has a point, to a point. Reversal film is more sensitive to exposure corrections than print film, so blown highlights become obvious with smaller levels of exposure change than print film. So, yeah, one needs to be cognizant of this when shooting slides. But it's still all about exposure, and I submit that, as I mentioned in my above post, if you take a pic of a bride's gown outdoors using the Sunny f/16 Rule, the gown should be correctly exposed and the highlights shouldn't be blown.