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	$37.50	12.503%	$42.1886	$42.19	$42.20	$42.20
Purchase #2	$166.67	10.614%	$184.3604	$184.36	$184.35	$184.40
Purchase #3	$48.64	9.518%	$53.2696	$53.27	$53.25	$53.30
Purchase #4	$184.86	7.556%	$198.828	$198.83	$198.85	$198.80
Purchase #5	$188.72	11.608%	$210.6266	$210.63	$210.65	$210.60
Purchase #6	$42.62	13.327%	$48.3	$48.30	$48.30	$48.30
Purchase #7	$74.95	11.497%	$83.567	$83.57	$83.55	$83.60
7 purchase totals =	$743.96		$821.1402	$821.15	$821.15	$821.20