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	$17.27	7.107%	$18.4974	$18.50	$18.50	$18.50
Purchase #2	$250.90	12.182%	$281.4646	$281.46	$281.45	$281.50
Purchase #3	$63.74	3.415%	$65.9167	$65.92	$65.90	$65.90
Purchase #4	$167.38	7.173%	$179.3862	$179.39	$179.40	$179.40
Purchase #5	$97.56	11.405%	$108.6867	$108.69	$108.70	$108.70
Purchase #6	$182.70	12.336%	$205.2379	$205.24	$205.25	$205.20
Purchase #7	$49.73	3.200%	$51.3214	$51.32	$51.30	$51.30
7 purchase totals =	$829.28		$910.5109	$910.52	$910.50	$910.50