It can't hurt to rate a 500T stock at 320 or 400 ASA if practical, because it gives you a little more leeway for accidental underexposure.
In terms of how much you can underexpose the stock, it depends on how much underexposure you are talking about. You have to be specific, you have to know the light levels you are talking about.
The other thing to remember is that the "correct" exposure isn't always at key level. In other words, let's say you are in a nightclub with colored lighting. If you exposed that light fully to what your incident meter told you, it would actually look overexposed, too bright. Because to look "correct" and saturated, a colored spot light has to be a bit dimmer than full exposure, plus the overall ambience of the nightclub should feel under key exposure to be believable. So if you have a lens that only opens to f/2.8 and your meter tells you that the colored lights hitting the people are at f/2.0, or between an f/1.4 and f/2.0, then shooting at an f/2.8 would actually look about right, exposure-wise.
Same goes for a face being lit by a candle flame -- unless it was very close to the flame, it would probably be falling-off a bit in brightness, maybe it should look about a stop under key.
So you have some leeway in low-light scenes because objects may naturally be dimmer than full exposure anyway.
However, you do have to know just how much underexposed the subject is, the film is not magical, it needs a certain amount of light to get an image, and if the image is too dark for your tastes, then you'll be lightening it up in post -- which will cause the grain or noise to become more visible.
There's also a difference between overall underexposure that you need to correct up, and an image that you don't make brighter in post but you want to know where shadow detail falls to black.
If you lift everything, you are basically rating the stock faster -- for example, if everything is a stop underexposed and you lift it up in post, it's basically like rating 500T at 1000 ASA. So asking how fast you can rate a stock and get away with it, grain-wise, is a different question than how dark you can make areas in the frame but leave the stock at a 500 ASA rating.
It's the difference between dynamic range (the exposure range captured) and latitude (the range you can make corrections). Latitude is much narrower than dynamic range.
