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	$250.80	11.473%	$279.5743	$279.57	$279.55	$279.60
Purchase #2	$204.33	9.209%	$223.1467	$223.15	$223.15	$223.10
Purchase #3	$241.34	8.515%	$261.8901	$261.89	$261.90	$261.90
Purchase #4	$40.40	5.571%	$42.6507	$42.65	$42.65	$42.70
Purchase #5	$147.77	8.581%	$160.4501	$160.45	$160.45	$160.50
Purchase #6	$110.71	10.146%	$121.9426	$121.94	$121.95	$121.90
Purchase #7	$115.71	13.630%	$131.4813	$131.48	$131.50	$131.50
7 purchase totals =	$1,111.06		$1,221.1358	$1,221.13	$1,221.15	$1,221.20