If you have the time for a detailed metering technique, then I would not rate it any slower than 1/3 stop below box speed for average or mixed shooting conditions; and only as an assumption that your density threshold will probably not exactly match the manufacturer's; NOT as a *blanket* exposure compensation.
There is no need. All it does is slightly lower the overall contrast of the film for the entire roll (which you just may want). You gain what I feel is an unnecessary and distracting amount of shadow detail and texture at the expense of saturation and punch, and can get unnatural tonalities and increased grain (though probably not much unless you go over more than a stop).
If you use an in-camera (reflected) averaging meter, or some derivation thereof, be it center weighted, matrix, etc., then the problems come in. Lack of time/lack of desire to make detailed meter readings or just plain poor metering practice (AKA not knowing what the heck a reflected meter really tells you) is why many people half rate their film to "cover" themselves in the shadows.
It's FAR from the worst practice in the world, and won't ruin anything. It's almost always better to be over than under on negs, but if you have the time, you might as well just take a few seconds longer with your meter and nail it without gimmicks.
The reason so many people's films look better when they downrate them is because they usually base their exposures off of an in-camera reflected reading, which more often than not in daylight will give a negative with less density than is ideal for the print desired (AKA "underexposed"). Similarly, people shooting in low light will often "overexpose". These people really should know that they should use *individual*, shot-by-shot EC. This means to open up from the meter reading in many circumstances, and close down in others, instead of simply resorting to a blanket exposure compensation by lying to their meter (or they should meter off a grey card or use an incident meter).
I don't know...do you want to play it safe so you can shoot more care freely and focus more on other things, or do you want to use your skill to simply nail it, which in turn makes your printing experience and results much more pleasant? Both work. By your choice of equipment, however, I can't imagine why you wouldn't just use a point and shoot if you prefer the former.
The final print is all that really matters to *anyone else* in the end, but your experience and thought pattern while shooting are also important, and should be fun and challenging for you. That's a large part of the draw of a manual camera these days in the midst of a flood of the best point and shoots the world has ever seen...isn't it?
Quite honestly, when using small format, I totally wing it with box rating and BDE as a guide, since winging it is what I think small format is all about anyhow.
One fools opinion...doesn't mean I'm right or wrong. Just another angle to consider. As I mentioned before, I think you will be fine following my advice, OR the advice of anyone else in this thread.