Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
Case	Tag   Price	Sales   Tax %	Raw Total   Incl. Sales Tax	Rounded   to Penny	Rounded   to Nickel	Rounded   to Dime
Purchase #1	$28.40	6.729%	$30.311	$30.31	$30.30	$30.30
Purchase #2	$250.65	11.800%	$280.2267	$280.23	$280.25	$280.20
Purchase #3	$28.58	12.770%	$32.2297	$32.23	$32.25	$32.20
Purchase #4	$152.77	4.527%	$159.6859	$159.69	$159.70	$159.70
Purchase #5	$97.31	5.346%	$102.5122	$102.51	$102.50	$102.50
Purchase #6	$93.78	10.183%	$103.3296	$103.33	$103.35	$103.30
Purchase #7	$227.86	9.866%	$250.3407	$250.34	$250.35	$250.30
7 purchase totals =	$879.35		$958.6358	$958.64	$958.70	$958.50