Underexposing transparencies is fine if all you want to do is project them, for example. But as an underexposed frame of Velvia, for instance, represents one of the toughest scanning jobs for some of the scanners many of us use such as the Epsons and Canons, if you intend to scan the processed film, it has in my view become more important than ever to nail the exposure you want. Underexposing some scenes will only make it that much harder for many scanners to extract usable shadow detail than it already is.
It's certainly true that transparency film has little tolerance for overexposure however in general I incident meter and try to get the exposure bang on or even up to a half a stop over. I tend to use older fully mechanical cameras most of the time that can often, at best, set half apertures and only full shutter speeds. So in practice this means that, where I take a reading that is not bang on, f/8, for example, I may tend to set the f stop or shutter speed combo that is closest to that but slightly over, not slightly under. The exposure as metered then falls somewhere between a half stop and bang on, and this usually works out OK. Of course, some subjects demand shadow, or highlight, detail, at all costs--so you take your pick and let the other fall where it will--but that is taken as read.
Personally, now that little colour output from film is done in a wet darkroom, I don't understand why more people don't use transparency. True, it is harder to expose correctly, but most of us members here at RFF would like to think we are up to the challenge of this tecnhically, yes? I certainly find it more satisfying to get right alhough I cheerfully admit others might see it as a chore. It's also easier to match colour accuracy of a scan from the transparency frame, and you can view them with a lightbox or projector with superb quality.
There are certainly some subjects such as weddings better suited to colour negative if you must shoot film but generally speaking a positive just looks so much better. And please don't talk to me about cost of reversal film or processing--come Down Under for a while and use E6 and then, perhaps, you would get a sympathetic reception from me about the expense...many of you really do not know how good you actually have it.
Regards
Brett