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	$30.56	4.359%	$31.8921	$31.89	$31.90	$31.90
Purchase #2	$162.50	6.555%	$173.1519	$173.15	$173.15	$173.20
Purchase #3	$67.52	7.629%	$72.6711	$72.67	$72.65	$72.70
Purchase #4	$26.47	9.304%	$28.9328	$28.93	$28.95	$28.90
Purchase #5	$230.54	3.745%	$239.1737	$239.17	$239.15	$239.20
Purchase #6	$177.58	4.386%	$185.3687	$185.37	$185.35	$185.40
Purchase #7	$122.61	12.773%	$138.271	$138.27	$138.25	$138.30
7 purchase totals =	$817.78		$869.4613	$869.45	$869.40	$869.60