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	$18.33	12.243%	$20.5741	$20.57	$20.55	$20.60
Purchase #2	$73.40	6.313%	$78.0337	$78.03	$78.05	$78.00
Purchase #3	$37.76	11.608%	$42.1432	$42.14	$42.15	$42.10
Purchase #4	$96.33	4.131%	$100.3094	$100.31	$100.30	$100.30
Purchase #5	$43.36	12.102%	$48.6074	$48.61	$48.60	$48.60
Purchase #6	$206.95	6.669%	$220.7515	$220.75	$220.75	$220.80
Purchase #7	$100.73	8.775%	$109.5691	$109.57	$109.55	$109.60
7 purchase totals =	$576.86		$619.9884	$619.98	$619.95	$620.00