I think that fitday rounds off the display, but uses the actual number in its calculations, so the total you see at the top of the screen is off by no more than .5 gram.

For instance, I just looked at my fitday totals for yesterday. The total carb count at the top of the screen is 17. If I total up the carbs displayed for each food, it adds up to 18. But if I check the detailed info for each food and add up the exact carb counts myself, it's 17.3. So I think the rounding off is just done for the display, it's not calculating the totals with those rounded numbers.

I have some problems with fitday -- it's cumbersome to use, times out too quickly, and I really really wish we could build custom foods by combining ingredients, rather than just entering the totals -- but I do think that, as long as the carb counts for the ingredients are correct, it is calculating accurately.