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	$29.46	6.357%	$31.3328	$31.33	$31.35	$31.30
Purchase #2	$140.61	12.483%	$158.1623	$158.16	$158.15	$158.20
Purchase #3	$135.41	11.698%	$151.2503	$151.25	$151.25	$151.30
Purchase #4	$249.50	4.114%	$259.7644	$259.76	$259.75	$259.80
Purchase #5	$37.29	7.754%	$40.1815	$40.18	$40.20	$40.20
Purchase #6	$17.92	5.518%	$18.9088	$18.91	$18.90	$18.90
Purchase #7	$28.28	12.360%	$31.7754	$31.78	$31.80	$31.80
7 purchase totals =	$638.47		$691.3755	$691.37	$691.40	$691.50