I might be reading this wrong but I did a quick search and found this here: http://thesandtrap.com/b/rules/whats_your_handicap
It doesn't say anything about rounding the number after calculating the differential for a round.
The reason I keep an informal one is so I can tell someone I might play with what my informal handicap is so they get an idea of how I play and also as I measuring stick when playing with my friends who have an official handicap.
Maybe he doesn't say anything specific about rounding, but look at his example ...
If you shoot 83 on a course with course and slope ratings of 71.5 and 123.4, your differential for that round is (83 - 71.5) * (113 / 123.4) = 10.5. Calculate this differential for each round.
If you punch that into your calculator, you don't get 10.5, you get 10.53079417. So that is clarly the point where the rounding comes in.
Keep in mind, though, that if you don't do that, you are going to be affecting your number by only the tiniest, tiniest amount. And perhaps not even at all. I wouldn't worry about it too much. :)