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	$18.96	8.144%	$20.5041	$20.50	$20.50	$20.50
Purchase #2	$139.25	12.289%	$156.3624	$156.36	$156.35	$156.40
Purchase #3	$138.54	6.609%	$147.6961	$147.70	$147.70	$147.70
Purchase #4	$171.94	5.450%	$181.3107	$181.31	$181.30	$181.30
Purchase #5	$60.33	7.438%	$64.8173	$64.82	$64.80	$64.80
Purchase #6	$79.54	12.737%	$89.671	$89.67	$89.65	$89.70
Purchase #7	$131.31	8.855%	$142.9375	$142.94	$142.95	$142.90
7 purchase totals =	$739.87		$803.2991	$803.30	$803.25	$803.30