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	$248.92	10.370%	$274.733	$274.73	$274.75	$274.70
Purchase #2	$123.28	10.728%	$136.5055	$136.51	$136.50	$136.50
Purchase #3	$42.89	6.116%	$45.5132	$45.51	$45.50	$45.50
Purchase #4	$186.42	13.104%	$210.8485	$210.85	$210.85	$210.80
Purchase #5	$27.27	4.669%	$28.5432	$28.54	$28.55	$28.50
Purchase #6	$82.73	8.596%	$89.8415	$89.84	$89.85	$89.80
Purchase #7	$66.55	11.809%	$74.4089	$74.41	$74.40	$74.40
7 purchase totals =	$778.06		$860.3938	$860.39	$860.40	$860.20