It's a grey area really...
On a serious note, every printer is obviously different, so the colours will vary with the same readings (you've heard it all before). I'm not overly familiar with how the process works with smaller detailed print, particularly for business cards, but where I work (we do large format), we're often asked to match according to physical samples, to which we can get close to by using process. But then the colour might change again once sealed etc. Matte finishes are generally better for text, whilst gloss is better for pictures.
The problem being, colour matching is so ambiguous, a printer could make your grey darker or brighter, but unless they have something to match to, you won't get the desired colour. There are also things like material colour, thickness and ink density to take into consideration too. Again it might just be where I work, but it's often the case that certain materials appear slightly whiter than others.
In some situations, the person doing the printing might create a swatch and get you to approve one of the colours. I think it really depends on where you get your printing done, because certain companies have different methods and some might charge you for the above process, whereas a few won't (they'll just charge you more for something else).
You could amend your artwork to make the grey appear bolder (altering the surrounding colours sometimes works), but this shouldn't really be necessary. You'd also be guessing by altering the black percentage, although you might be pleased with the result, it'd be a gamble. I'll be interested to see the views of someone who does detailed print, I'm still relatively new to the print scene.
