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	$24.37	12.784%	$27.4855	$27.49	$27.50	$27.50
Purchase #2	$244.32	7.205%	$261.9233	$261.92	$261.90	$261.90
Purchase #3	$221.41	13.204%	$250.645	$250.65	$250.65	$250.60
Purchase #4	$11.39	11.240%	$12.6702	$12.67	$12.65	$12.70
Purchase #5	$10.57	6.144%	$11.2194	$11.22	$11.20	$11.20
Purchase #6	$192.40	11.136%	$213.8257	$213.83	$213.85	$213.80
Purchase #7	$116.38	9.592%	$127.5432	$127.54	$127.55	$127.50
7 purchase totals =	$820.84		$905.3123	$905.32	$905.30	$905.20