Hello Ocelot,
I am assuming you have a site that has affiliate links and you want to keep Google from crawling those affiliate links. If I am wrong, please let me know. Going forward with that assumption then...
That is one way to do it. So perhaps you first send all of those links through a redirect via a folder called /out/ or /links/ or whatever, and you have blocked that folder in the robots.txt file. Correct? If so, this is how many affiliate sites handle the situation.
I would not rely on rel nofollow alone, though I would use that in addition to the robots.txt block.
There are many other ways to handle this. For instance, you could make all affilaite links javascript links instead of href links. Then you could put the javascript into a folder called /js/ or something like that, and block that in the robots.txt file. This works less and less now that Google Preview Bot seems to be ignoring the disallow statement in those situations.
You could make it all the same URL with a unique identifyer of some sort that tells your database where to redirect the click. For example:
www.yoursite.com/outlink/mylink#123
or
www.yoursite.com/mylink?link-id=123
In which case you could then block /mylink in the robots.txt file and tell Google to ignore the link-ID parameter via Webmaster Tools.
As you can see, there is more than one way to skin this cat. The problem is always going to be doing it without looking like you're trying to "fool" Google - because they WILL catch up with any tactic like that eventually.
Good luck!
Everett