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	$16.78	6.673%	$17.8997	$17.90	$17.90	$17.90
Purchase #2	$152.63	10.222%	$168.2318	$168.23	$168.25	$168.20
Purchase #3	$62.25	13.159%	$70.4415	$70.44	$70.45	$70.40
Purchase #4	$188.89	13.680%	$214.7302	$214.73	$214.75	$214.70
Purchase #5	$154.88	8.215%	$167.6034	$167.60	$167.60	$167.60
Purchase #6	$241.37	9.322%	$263.8705	$263.87	$263.85	$263.90
Purchase #7	$243.98	5.797%	$258.1235	$258.12	$258.10	$258.10
7 purchase totals =	$1,060.78		$1,160.9006	$1,160.89	$1,160.90	$1,160.80