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	$88.77	11.832%	$99.2733	$99.27	$99.25	$99.30
Purchase #2	$197.42	13.286%	$223.6492	$223.65	$223.65	$223.60
Purchase #3	$78.57	6.428%	$83.6205	$83.62	$83.60	$83.60
Purchase #4	$64.86	5.372%	$68.3443	$68.34	$68.35	$68.30
Purchase #5	$18.65	9.445%	$20.4115	$20.41	$20.40	$20.40
Purchase #6	$227.62	11.547%	$253.9033	$253.90	$253.90	$253.90
Purchase #7	$89.85	5.228%	$94.5474	$94.55	$94.55	$94.50
7 purchase totals =	$765.74		$843.7495	$843.74	$843.70	$843.60