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	$100.73	8.328%	$109.1188	$109.12	$109.10	$109.10
Purchase #2	$128.51	3.702%	$133.2674	$133.27	$133.25	$133.30
Purchase #3	$159.40	13.253%	$180.5253	$180.53	$180.55	$180.50
Purchase #4	$9.34	10.677%	$10.3372	$10.34	$10.35	$10.30
Purchase #5	$19.71	8.150%	$21.3164	$21.32	$21.30	$21.30
Purchase #6	$54.63	8.414%	$59.2266	$59.23	$59.25	$59.20
Purchase #7	$25.72	12.376%	$28.9031	$28.90	$28.90	$28.90
7 purchase totals =	$498.04		$542.6948	$542.71	$542.70	$542.60