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	$242.32	3.336%	$250.4038	$250.40	$250.40	$250.40
Purchase #2	$223.27	7.803%	$240.6918	$240.69	$240.70	$240.70
Purchase #3	$80.74	12.643%	$90.948	$90.95	$90.95	$90.90
Purchase #4	$55.88	5.867%	$59.1585	$59.16	$59.15	$59.20
Purchase #5	$222.39	3.718%	$230.6585	$230.66	$230.65	$230.70
Purchase #6	$28.62	9.211%	$31.2562	$31.26	$31.25	$31.30
Purchase #7	$71.70	11.571%	$79.9964	$80.00	$80.00	$80.00
7 purchase totals =	$924.92		$983.1132	$983.12	$983.10	$983.20