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	$29.50	4.442%	$30.8104	$30.81	$30.80	$30.80
Purchase #2	$73.63	7.713%	$79.3091	$79.31	$79.30	$79.30
Purchase #3	$141.47	12.636%	$159.3461	$159.35	$159.35	$159.30
Purchase #4	$93.49	6.270%	$99.3518	$99.35	$99.35	$99.40
Purchase #5	$229.56	4.667%	$240.2736	$240.27	$240.25	$240.30
Purchase #6	$31.96	10.545%	$35.3302	$35.33	$35.35	$35.30
Purchase #7	$107.39	4.207%	$111.9079	$111.91	$111.90	$111.90
7 purchase totals =	$707.00		$756.3291	$756.33	$756.30	$756.30