I personally do it a lot. If it looks ok, then it must be ok. Changing the leading in a proportional increment is a good practice in running text but in this case I would agree with anything that works.

Héctor

As long as you keep the spacing consistant throughout the stationary design it shouldn't be a problem.

I agree with rs_donsata -- If it looks good, it's good.

I really think that the guide has to be the appearance other than any set spacing rule in cases where it isn't running text, but rather separate groupings of information. It must make sense optically instead of numerically, in other words. With a business card, where several related but distinctly different groupings of information have to be conveyed, I don't even think that the driving concern should be consistency. The goal is to move the reader through the info at the pace YOU intend.

With the blocks that you suggest, the space between numerals and email address might appear more due to the absence of descenders on aligning numerals, unless like the link you provided you have an uppercase postcode. If the last line of the address contains upper and lower case you could work on a percentage of leading, you could, for example, specify half line space after address block and third line space after the numerals. However, because aligning numerals are the same height you could also consider greater leading for the two lines of numerals.
Tim

Thank you all for your imput.
Actually, the column of text has 5 blocks total, all consisting of two lines: so it is symmetrical already.
Tim, I have to confess that I am doing something a little against the rules: I am using old-style numerals, even though it's not running text. Would that be a mortal typographic sin in a stationery system?
I know you're supposed to use lining figures when you stack lines of numbers, but I am using small caps in two blocks, and I hate the way lining figures look with them (I have T and F for telephone and fax in small caps, plus IVA and CF in the last block also, which are the Italian equivalent of VAT numbers). The CF line is actually a mix of numbers and letters, and I think small caps + old style figure look great in this case.
So, with 10 lines of text stacked in blocks of 2, the column is quite tall: I like the way it looks with line spaces of reduced leading instead of simple hard returns. It adds a certain variety, and the column looks much more compact.
I think I'll opt for that solution, reducing the leading by half in all the space lines since my numbers do have ascenders and descenders.

There are no "rules" about using aligning and non-aligning, as earlier posts have said if it looks good, it is good, essentially you are trying to present your client's information in a useful* and attractive way. Personally I prefer the look of old style numerals.

*by useful I mean that the recipients of the letter/business card can pick out the details they might need.

Tim