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	$37.42	5.562%	$39.5013	$39.50	$39.50	$39.50
Purchase #2	$94.28	11.724%	$105.3334	$105.33	$105.35	$105.30
Purchase #3	$172.36	9.603%	$188.9117	$188.91	$188.90	$188.90
Purchase #4	$152.36	13.414%	$172.7976	$172.80	$172.80	$172.80
Purchase #5	$53.85	7.124%	$57.6863	$57.69	$57.70	$57.70
Purchase #6	$96.51	13.490%	$109.5292	$109.53	$109.55	$109.50
Purchase #7	$161.55	3.474%	$167.1622	$167.16	$167.15	$167.20
7 purchase totals =	$768.33		$840.9217	$840.92	$840.95	$840.90