What you're asking is not possible. Anytime you round a number you're going to introduce error, how much error will depend on how much you're rounding. Add enough errors together and your total will always be off from the "expected" total (in this case 100%).
in reply to accurately rounding numbers for percentages
The only way around this is to go ahead and round the individual entries to whole numbers for display, but when calculating the total don't add the rounded entries, add the unrounded entries, and then round the result for display, if you want.