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	$35.92	11.244%	$39.9588	$39.96	$39.95	$40.00
Purchase #2	$31.54	12.118%	$35.362	$35.36	$35.35	$35.40
Purchase #3	$86.64	4.838%	$90.8316	$90.83	$90.85	$90.80
Purchase #4	$98.97	12.391%	$111.2334	$111.23	$111.25	$111.20
Purchase #5	$55.26	6.442%	$58.8198	$58.82	$58.80	$58.80
Purchase #6	$31.82	4.614%	$33.2882	$33.29	$33.30	$33.30
Purchase #7	$106.99	9.645%	$117.3092	$117.31	$117.30	$117.30
7 purchase totals =	$447.14		$486.8030	$486.80	$486.80	$486.80